dt-bindings: arm: renesas: Move 'renesas,prr' binding to its own doc

In preparation to convert board-level bindings to json-schema, move
various misc SoC bindings out to their own file.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>