SWD: Add resistors to protect against accidental pin swapping

[matthiasbock]

It can easily happen, that the user accidentally attaches debug cables to the wrong pins:

• an SWDIO cable (TDI or TDO) to the SWDCLK pin
  or
• the TDI cable to the TDO pin

This way output pins may get shortcircuited:

• TDI and SWDIO
  or
• TDI and SWDCLK

("Test Data In" (TDI) refers the JTAG state machine perspective: Data into the state machine)

To prevent this, additional resistors should be added:

• between TDI and SWDIO
  and
• between SWDCLK cable and pin