Move bhnd_nexus driver out into sys/mips/broadcom to support MIPS-specific behavior.

[landonf]

Summary

• Move bhnd/bcma/siba nexus drivers out into sys/mips/broadcom.
• Add MIPS/BCM4706-specific workaround to bhnd_nexus_is_hw_disabled() -- the BCM4706 low-cost package leaves secondary GMAC cores floating.

[adrian]

if michael is okay with it then I'm okay with it

[mizhka]

Of course I like it. More broadcom features. I hope to see ARM BHND support in future :) I'll cross fingers before start rebase