boards: nrf52_pca10040: 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: I2785441c286b95fd77e636e9bf3d68bc9f5a1acc
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>