boards: adi: Add max32690fthr

Adds a new board definition for the ADI MAX32690FTHR board. This board
uses the same SoC as the MAX32690EVKIT board, but with a smaller feather
form factor.

Tested on hardware with the following samples:
- samples/hello_world
- samples/basic/blinky
- samples/basic/threads
- samples/subsys/shell/shell_module
- samples/subsys/input/input_dump

Signed-off-by: Maureen Helm <maureen.helm@analog.com>