dts: nordic: Use NRF_DEFAULT_IRQ_PRIORITY instead of hard-coded values

Instead of hard-coded values for interrupt priorities in DT nodes, use
a macro that additionally can be overridden at the board or application
level. This allows, for instance, changing the interrupt priorities for
all but one peripheral without using a lengthy overlay file.

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