dt-bindings: power: rcar-sysc: Convert to json-schema

Convert the Renesas R-Car System Controller Device Tree binding
documentation to json-schema.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Rob Herring <robh@kernel.org>