drivers: pinctrl: nrf: Optimize access to gpd service

Request and release global power domain only once during setup
of pins. Request and release involves communication over IPC and
it should be avoided if possible. For example if there are 4 pins
(like in UART) where GPD is requested we can limit number of
request/release operations fourfold.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>