dt-bindings: mfd: Convert Allwinner legacy PRCM bindings to schemas

The Allwinner SoCs have a legacy set of bindings (and a drivers to
support it in Linux) to support the PRCM unit found in most recent SoCs.

Now that we have the DT validation in place, let's split into separate file
and convert the device tree bindings for those controllers to schemas, and
mark them all as deprecated.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Rob Herring <robh@kernel.org>