[bhnd] Clean up bhnd resource handling and inherited bus methods

To facilitate use by SoC implementors working with bhnd-inheriting fdt/nexus
drivers:

• Splits bhnd_bus method implementations into generic bus implementations (bhnd_bus_generic) and generic bhnd(4) driver implementations (bhnd_generic)
• Simplifies bhnd resource handling, allowing bhnd bus implementations to support bhnd resource activation by implementing the standard BUS_* resource APIs and BHND_BUS_ACTIVATE_RESOURCE().

Submitted by: Landon Fuller <landonf@landonf.org>
Differential Revision: https://reviews.freebsd.org/D5758