dt-bindings: powerpc: Convert fsl/pmc.txt to YAML

dt-bindings: powerpc: Convert fsl/pmc.txt to YAML

This patch rewrites pmc.txt into YAML format. Descriptive texts are
expanded or shortened in a few places to better fit today's conventions.

The list of compatible strings (and combinations of them) is based on
existing device trees in arch/powerpc as well as compatible strings
already mentioned in the plain-text version of the binding.

One thing I didn't handle are soc-clk@... nodes as seen in
arch/powerpc/boot/dts/fsl/pq3-power.dtsi. They are also ignored
by Linux drivers.

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
Link: https://lore.kernel.org/r/20250417-fslpmc-yaml-v3-1-b3eccd389176@posteo.net
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>