mcp23xxx: support for Microchip MCP23 family of IO extenders

[balazsgrill]

This driver supports the following I2C and SPI IO extenders:

• MCP23016
• MCP23017/MCP23S17
• MCP23018/MCP23S18
• MCP23008/MCP23S08
• MCP23009/MCP23S09

The driver is still under development, Interrupt support is not yet implemented.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[maruel]

Sorry for the delay. I can't approve because both the CLA checker and Travis failed. For the CLA checker, see the instructions above.

For travis, if you click through, you'll end up on https://travis-ci.org/google/periph/jobs/653761040 and scrolling a bit you'll find the red text:

Check Code is well formatted
The command "if [[ $TRAVIS_GO_VERSION != 1.7.6 ]]; then echo 'Check Code is well formatted'; ! gofmt -s -d . | read; fi" exited with 1.

Basically, run gofmt -s on the new files and it should do a small number of modifications.

[balazsgrill]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[codecov-io]

Codecov Report

Merging #433 into master will decrease coverage by 0.14%. The diff coverage is 55.04%.

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
- Coverage   63.51%   63.36%   -0.15%     
==========================================
  Files         117      120       +3     
  Lines       12187    12425     +238     
==========================================
+ Hits         7740     7873     +133     
- Misses       4249     4344      +95     
- Partials      198      208      +10

Impacted Files	Coverage Δ
experimental/devices/mcp23xxx/pins.go	33.33% <33.33%> (ø)
experimental/devices/mcp23xxx/mcp23xxx.go	54.8% <54.8%> (ø)
experimental/devices/mcp23xxx/registers.go	92% <92%> (ø)
devices/lepton/lepton.go	88.41% <0%> (+1.21%)	:arrow_up:

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[maruel]

gohci

[maruel]

Could you fix some of the comments? In practice the copyright one is the one I care the most about.

[maruel]

gohci

[maruel]

Thanks!