dt-bindings: interrupt-controller: Convert st,spear3xx-shirq to DT schema

Convert the SPEAr3xx Shared interrupt controller binding to schema
format. It's a straight-forward conversion of the typical interrupt
controller.

Link: https://lore.kernel.org/r/20250505144851.1293180-1-robh@kernel.org


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>