Convert the Open PIC interrupt controller binding to schema format.
While the Linux kernel supports the "open-pic" compatible, that's not
used in any upstream .dts file. It used for "device_type" though. Add
"fsl,mpic" compatible which was not documented.
Link: https://lore.kernel.org/r/20250505144809.1291619-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
