drivers: pinctrl_nrf: Add missing break statements

This is a follow-up to commit fd763312.

For some reason the above commit added several switch cases without
required break statements. In effect, the same pin could get assigned
to multiple signal lines in QDEC or QSPI peripherals if not all pins
were defined for them in devicetree, and consequently these peripherals
could not work properly.

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