dts: Adding priority cell to Intel's IOAPIC IRQ controllers descriptor

Zephyr exposes the ability to set IRQ priority on IOAPIC.
To keep compatibility with Linux, let's add the priority at the end
after sense. So imported dtsi with interrupt-cells set to 2 will work as
usual.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>

dts/bindings/interrupt-controller/intel,ioapic.yaml

@@ -28,4 +28,5 @@ properties:
 "#cells":
   - irq
   - sense
+  - priority
 ...