bhnd: Implement bhnd(4) platform device registration.
Add bhnd(4) API for explicitly registering BHND platform devices (ChipCommon,
PMU, NVRAM, etc) with the bus, rather than walking the newbus hierarchy to
discover platform devices. These devices are now also refcounted; attempting
to deregister an actively used platform device will return EBUSY.
This resolves a lock ordering incompatibility with bwn(4)'s firmware loading
threads; previously it was necessary to acquire Giant to protect newbus access
when locating and querying the NVRAM device.
Approved by: adrian (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12392