boards: bbc_microbit: Add defines for all edge connector pins

The schematics of the BBC micro:bit are hard to find, and its official
web-site doesn't document the mapping of the edge connector pins
numbers the nRF51 GPIO pin numbers. Provide helpful defines for this
in the board.h file.

Change-Id: I52ce1d61558703b6aa5a5d073ccd222f27fa8760
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>