ns9xxx: add support for irq priorisation

prio2irq(0) is the highest prioritized interrupt.  For now there is no
binary change intended.

If you intend to change the priorisation, you have to assert:

	∀ x ∈ {0, 1, ⋯ 31}: (irq2prio ∘ prio2irq)(x) = x

Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>