drivers: can: nrf: disable pin retention if GPD is enabled

When GPD is enabled, the pinctrl driver enables pin retention if the
peripheral is in the FAST_ACTIVE1 domain. This seems to break CAN, so
make sure retention is not active.

Investigations on why this is needed are ongoing, however, this makes
CAN functional again with some minor changes.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>