[mips/broadcom] Add initial support for Broadcom MIPS processor

Relies on BHND(4) driver.

There files contains machine-dependent code for Broadcom MIPS processor and
provide UART driver.

This is a work in progress; it and the current bhnd code is enough to boot
on the ASUS RT-N16 and RT-N53 platforms.

Submitted by: Michael Zhilin <mizhka@gmail.com>
Differential Revision: https://reviews.freebsd.org/D6251