Move phy-rockchip-dphy-rx0 bindings to Documentation/devicetree/bindings/phy

Verified with:
make ARCH=arm64 dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/phy/rockchip-mipi-dphy-rx0.yaml

Signed-off-by: Helen Koike <helen.koike@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>