gpio: nrf5x: Add support for GPIOTE and GPIO callbacks

To support interrupt-based GPIO callbacks in the nRF5x series one needs
to use the GPIOTE module, which has a series of channels that can be
programmed to trigger interrupts on level changes.
This commit adds basic support for the GPIOTE module within the nRF5x
GPIO driver, as well as callback support in order for basic buttons and
switches to work.

Change-Id: I9ae600f894372ad42b09a18cc38a90fc29abb0df
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>