[bhnd] Add BHND SoC support (only BCMA case)

This patch introduces support of BHND on SoC: nexus-to-bhnd drivers.

bhnd_soc is attached under nexus and responsible for all BHND-style calls
from bhnd(4) bus to parents.

bcma_nexus is bhnd(4) bus, attached to bhnd_soc and implement all
nexus-style behaviour of BHND.

Submitted by: Michael Zhilin <mizhka@gmail.com>
Reviewed by: Landon Fuller <landonf@landonf.org>
Differential Revision: https://reviews.freebsd.org/D6248