drm/sun4i: Add support for HDMI voltage regulator

Some boards have HDMI VCC pin connected to voltage regulator which may
not be turned on by default.

Add support for such boards by adding voltage regulator handling code to
HDMI driver.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
[Icenowy: change supply name to "hvcc"]
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904044053.15425-11-icenowy@aosc.io