drivers: gpio: nrfx: impl pm device runtime

Implement PM_DEVICE_RUNTIME for gpio_nrfx.c

PM is required to request/release the GPD domain if such exists
for a given GPIO instance. Without this, every single API call
will result in a requesting the GPD domain, which makes it slow
and unusable from ISRs, given GPD requires IPC. Now, user can
simply get the GPIO before using it, and put it once done,
calling the APIs from ISR while resumed.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>