Implement bhnd(4) platform device registration.

[landonf]

Summary

Add bhnd(4) API for explicitly registering BHND platform devices with the bus, rather than walking the newbus hierarchy to discover platform devices (ChipCommon, PMU, NVRAM, etc). These devices are now also refcounted; attempting to deregister an actively used platform device will return EBUSY.

This resolves a lock ordering issue 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.

Test Plan

Tested for regressions against both Broadcom MIPS SoCs and Wi-Fi chipsets.

[landonf]

Adopt a non-sleepable lock for the bhnd_service_registry; this allows use of
bhnd_service_registry_retain() while holding either an unbounded or bounded
sleep lock.