USB support
This revision adds USB (EHCI/OHCI/OTG, depending on SoC type) support for various Ralink/Mediatek SoCs.
Currently USB is not supported on MT7621, this will be a future addition.
A USB PHY driver is also included, so that we can properly initialize the USB PHY (e.g., clocks, resets, registers where needed), before attempting to initialize EHCI/OHCI/OTG functionality.