Replying to my own post:
Someone has already done what i'm proposing on an Archer C6, see here: adding a usb port to archer C6
I need the pinouts for the MT7621AT to try this. I have the datasheet for the MT7621 which identifies the pins (K1 &K2) for USB2 on the ball map, but what I need is the SOC pin outs so i know where to solder the D- and D+ data pins. Anyone got a schematic or link to one it would be appreciated.
I then need to modify the .dts file as shown here in post#6 dts mod
thanks...