dt-bindings: sun4i-drm: add HDMI VCC supply property for sun8i-dw-hdmi

Allwiner SoCs with DesignWare HDMI controller all come with a "HVCC"
pin, which is the VCC of HDMI part.

Add a supply property to specify HVCC's regulator in the device tree.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904044053.15425-10-icenowy@aosc.io