drivers: gpio: nRF5: Use lowest IRQ priority level as default

Fix the permitted range of IRQ priority to, 0 to 2, as
maximum one priority level is used, for zero latency IRQs
in nRF51, being an ARM Cortex-M0.

Update the Kconfig default GPIOTE IRQ priority level to the
lowest, for nRF51 a level of 2; for nRF52, a level of 5.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>