boards: bbc_microbit: Add button and LED definitions

For the basic samples to work, button and LED definitions are required
in board.h. These map the buttons and LEDs present on the board to
the corresponding GPIOs they are wired into.

In this particular case the LEDs are actually arranged in a 3x9 matrix,
with the GPIOs wired to the rows and columns of said matrix. An upcoming
patch will provide utility functions to access the matrix.

Change-Id: Icce93ee6a08ae28445c6dc4a41529105ac80f14a
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>