ARM64: dts: meson-gx: Add HDMI_5V regulator on selected boards

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 the regulator and set it always-on for now.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>