dt-bindings: sun6i-dsi: Add VCC-DSI supply property

Allwinner MIPI DSI controllers are supplied with SoC DSI
power rails via VCC-DSI pin.

Some board still work without supplying this but give more
faith on datasheet and hardware schematics and document this
supply property in required property list.

Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Merlijn Wajer <merlijn@wizzup.org>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191003064527.15128-5-jagan@amarulasolutions.com