dt-bindings: display: renesas,du: Add missing power-domains property

"make dtbs_check" complains:

    arch/arm/boot/dts/r8a7779-marzen.dt.yaml: display@fff80000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'
    arch/arm64/boot/dts/renesas/r8a77970-v3msk.dt.yaml: display@feb00000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'
    arch/arm64/boot/dts/renesas/r8a77970-eagle.dt.yaml: display@feb00000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'
    arch/arm64/boot/dts/renesas/r8a77980-condor.dt.yaml: display@feb00000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'
    arch/arm64/boot/dts/renesas/r8a77980-v3hsk.dt.yaml: display@feb00000: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'

Fix this by documenting the power-domains property.

Fixes: 99d66127

 ("dt-bindings: display: renesas,du: Convert binding to YAML")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/600d42256515f180bc84b72e8bdb5c5d9126ab62.1619700459.git.geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>