Implement a generic bhnd(4) device enumeration table API.

This defines a new bhnd_erom_if API, providing a common interface to device
enumeration on siba(4) and bcma(4) devices, for use both in the bhndb bridge
and SoC early boot contexts, and migrates mips/broadcom over to the new API.

This also replaces the previous adhoc device enumeration support implemented
for mips/broadcom.

Migration of bhndb to the new API will be implemented in a follow-up commit.

• Defined new bhnd_erom_if interface for bhnd(4) device enumeration, along with bcma(4) and siba(4)-specific implementations.
• Fixed a minor bug in bhndb that logged an error when we attempted to map the full siba(4) bus space (18000000-17FFFFFF) in the siba EROM parser.
• Reverted use of the resource's start address as the ChipCommon enum_addr in bhnd_read_chipid(). When called from bhndb, this address is found within the host address space, resulting in an invalid bridged enum_addr.
• Added support for falling back on standard bus_activate_resource() in bhnd_bus_generic_activate_resource(), enabling allocation of the bhnd_erom's bhnd_resource directly from a nexus-attached bhnd(4) device.
• Removed BHND_BUS_GET_CORE_TABLE(); it has been replaced by the erom API.
• Added support for statically initializing bhnd_erom instances, for use prior to malloc availability. The statically allocated buffer size is verified both at runtime, and via a compile-time assertion (see BHND_EROM_STATIC_BYTES).
• bhnd_erom classes are registered within a module via a linker set, allowing mips/broadcom to probe available EROM parser instances without creating a strong reference to bcma/siba-specific symbols.
• Migrated mips/broadcom to bhnd_erom_if, replacing the previous MIPS-specific device enumeration implementation.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7748