Initial import of Ralink/Mediatek MIPS SoC support #5

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.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5841