electric-eel: add MCP401XEV eval board connections

Enable the i2c driver and shell commands.
Add documentation for the connections between the STM32 eval board
and the MCP401XEV eval board.

BUG=None
TEST=Run the `i2c scan I2C1` command as detailed in doc/README.md
then use `i2c write` to adjust the digital potentiometer.

Change-Id: I613efc76c15376de8fc19b5c17ffaf55e985436c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/electric-eel/+/3733523
Tested-by: Paul Fagerburg <[email protected]>
Reviewed-by: Alexandru Stan <[email protected]>
4 files changed