dt-bindings: convert rockchip-drm.txt to rockchip-drm.yaml

convert the binding file rockchip-drm.txt to yaml format.
This was tested and verified on ARM and ARM64 with:
make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/display/rockchip/rockchip-drm.yaml
make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/display/rockchip/rockchip-drm.yaml

Changes since v2:
- add a missing ">" sign in maintainers list
- change the licens to GPL-2.0-only
- add "additionalProperties: false"
- change the commit message to conform that it was tested on both ARM and ARM64
Changes since v1:
- fixed worng sign-off
- fixed the path of the $id property to be the path of the yaml file

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20200121154314.3444-1-dafna.hirschfeld@collabora.com