dt-bindings: interrupt-controller: Convert fsl,mpic-msi to YAML

dt-bindings: interrupt-controller: Convert fsl,mpic-msi to YAML

As part of a larger effort to bring various PowerPC-related bindings
into the YAML world, this patch converts msi-pic.txt to YAML and moves
it into the bindings/interrupt-controller/ directory. The conversion may
necessarily be a bit hard to read because the binding is quite verbose.

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
Link: https://lore.kernel.org/r/20250611-msipic-yaml-v2-1-f2e174c48802@posteo.net
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>