ramips: mir3g dts define usb port Vcc volt regulator GPIO

ramips: mir3g dts define usb port Vcc volt regulator GPIO

Define USB port power on/off GPO as voltage regulator type instead of
exposing as a normal GPIO.

The GPO is now controlled by the USB driver via the voltage regulator
definition.  The regulator is of fixed output type (5V for USB) hence the
GPO switches power on/off to USB pin 1 (Vcc)

USB port power is enabled on driver load and disabled on driver unload.

Enable kernel support for fixed voltage regulator types on mt7621.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>