dt-bindings: powerpc: Drop duplicate fsl/mpic.txt

The chrp,open-pic binding schema already supports the "fsl,mpic"
compatible. Add a couple of missing properties and support for 4
"#interrupt-cells" to the chrp,open-pic binding, so fsl/mpic.txt can be
removed.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250814135157.2747346-2-robh@kernel.org


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