boards: RPi debug probe as general purpose microcontroller board

This is for using the Raspberry Pi Debug Probe as a general purpose
microcontroller board, not for using it in its original debugging role.

Product photo from https://www.raspberrypi.com/products/debug-probe/



Tested with the samples mentioned in the index.rst page.

Signed-off-by: Jonas Berg <jonas.s.t.berg@gmail.com>