[mips/broadcom] Move MIPS-specific bhnd(4) nexus drivers to
sys/mips/broadcom, and add MIPS/BCM4706-specific workaround to
bhnd_nexus_is_hw_disabled() -- the BCM4706 low-cost package leaves
secondary GMAC cores floating.

Reviewed by: mizhka
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D9499