arm64: dts: meson-axg: add vcc 5v regulator on the s400

This regulator is controlled by a GPIO and supplies various devices
on the board, such as the lineout codec, the usb supply or the lcd
controller.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>