drm/sun4i: sun6i_mipi_dsi: Add VCC-DSI regulator support

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

Add support for this supply pin by adding voltage
regulator handling code to MIPI DSI driver.

Tested-by: Merlijn Wajer <merlijn@wizzup.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191003064527.15128-6-jagan@amarulasolutions.com