Support pHAT with SSD1608

[mnishiguchi]

According to Pimoroni's catalog:

We started rolling out a higher resolution SSD1608 e-paper screen on Inky pHAT from late 2020 (250x122 pixels instead of 212x104!). These new screens require different drivers ...

As I looked into the Pimoroni's Python library, they are pretty different despite physically looking identical. The device variants can be found here in the source code.

So I ended up writing a dedicated HAL module for pHAT with SSD1608.

## Usage

# when type is `:phat_ssd1608`, `Inky.HAL.PhatSSD1608` module is used by default
Inky.start_link(:phat_ssd1608, :red, %{name: InkySample})

Notes

• I realized that the Display struct is not so useful for this particular model since only accent is changeable; others are static. So I decided to specify the display info as module attributes in Inky.HAL.PhatSSD1608 module.
• I tested with pHat SSD1608 Black and pHat SSD1608 Red running the "brief example" program in README.

[nyaray]

I like this overall, though I think that there could have been some deduplication in the nearly identical HAL files. Something for a later clean-up PR, I suppose.