boards: nrf52840_pca10056: 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.

Change-Id: Ia0be30cecd8c0ceabb495258480eb6f3ce13d3e5
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>