drivers: pinctrl_nrf: Add support for EXMIF pins

This is a follow-up to commit 45d827a5.

Although routing for those pins is configured via UICR, pinctrl still
needs to be involved so that it is possible to set desired drive mode
for them etc.
Add also the missing RWDS pin.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>