dts: arm: Put IRQ priority into the interrupt property

Rather than having a zephyr specific property we will encode the IRQ
priority as part of the interrupt property for ARM NVIC based interrupt
controllers.

Change-Id: I7d1489f0bffa7a6369f0622f748bb70dc83fa0cd
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>