arm64: dts: meson-gx: Add hdmi_5v regulator as hdmi tx supply

The hdmi_5v regulator must be enabled to provide power to the physical HDMI
PHY and enables the HDMI 5V presence loopback for the monitor.

Fixes: b409f625 ("ARM64: dts: meson-gx: Add HDMI_5V regulator on selected boards")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>