drivers: pinctrl: nrf: patch pin retain to follow lp mode

Pins can be configured to retain their config even when the power
domain they belong to is suspended. Update pinctrl_nrf to enable
retain only if the pin has been configured and is not in use
(pincnf is low-power/sleep), disable retain otherwise.

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