dt-bindings: soc: rockchip: Document RV1126 pmugrf

Document compatible string for Rockchip RV1126 pmugrf.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jagan Teki <jagan@edgeble.ai>
Link: https://lore.kernel.org/r/20220907160207.3845791-8-jagan@edgeble.ai


Signed-off-by: Heiko Stuebner <heiko@sntech.de>