dt-bindings: display: rockchip: add document for rk3066 hdmi

This patch adds a binding that describes the HDMI controller for
rk3066.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190330095639.14626-5-jbx6244@gmail.com