drm/meson: dw_hdmi: Add support for an optional external 5V regulator

On reference boards and derivatives, the HDMI Logic is powered by an external
5V regulator.
This regulator was set by the Vendor U-Boot, add optional support for it.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1512561268-29806-4-git-send-email-narmstrong@baylibre.com