[BHND/USB] Port of EHCI/OHCI support from ZRouter

This patch adds driver implementation for BHND USB core. Driver has been
imported from ZRouter project with small adaptions for FreeBSD 11.

Also it's enabled for BroadCom MIPS74k boards by default. It's fully tested
on Asus boards (RT-N16: external USB, RT-N53: USB bus between SoC and WiFi
chips).

Reviewed by: adrian (mentor), ray
Approved by: adrian (mentor)
Obtained from: ZRouter
Differential Revision: https://reviews.freebsd.org/D7781