arm: Add build time consistency check for irq priority defines

We need to make sure that __NVIC_PRIO_BITS & CONFIG_NUM_IRQ_PRIO_BITS
are set to the same value.  Add a simple build time check to ensure
this is the case.  This is to catch future cases of issues like
ZEP-2243.  This is a stop gap til we resolve ZEP-2262, which covers use
of both __NVIC_PRIO_BITS & CONFIG_NUM_IRQ_PRIO_BITS.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>