drivers: gpio: nrf: Remove CONFIG_GPIO_NRF_P0 and CONFIG_GPIO_NRF_P1

Remove Kconfig options for enabling device instances in favor of
taking that information only from device tree. Prior to that
change there was a mix of devicetree and Kconfig.

Bring back use of CONFIG_GPIO_NRF_INIT_PRIORITY.

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