dt-bindings: display: sunxi: add DT binding for Allwinner H6 DW HDMI

The Allwinner H6 SoC uses a v2.12a DesignWare HDMI controller, with
dedicated CEC and HDCP clocks added; the PHY connected is a standard
DesignWare HDMI PHY.

Add binding for it.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
[added HDCP clock and reset]
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181104182705.18047-19-jernej.skrabec@siol.net