dt-bindings: add used but undocumented rockchip grf compatible values

There are some more General Register Files used in devicetree files
already, but as of now undocumented in the binding document, fix that.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>