arm: dts: mt7623: enable the usb device on the mt7623n rfb

All versions of the mt7623n RFB have an USB port so enable the device.
There is a gpio that gets used to power up the port supply. Add support
for this gpio using the fixed-regulator driver.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>