Migrate bhndb(4) to the new bhnd_erom API.

Adds support for probing and initializing bhndb(4) bridge state using
the bhnd_erom API, ensuring that full bridge configuration is available
*prior* to actually attaching and enumerating the bhnd(4) child device,
allowing us to safely allocate bus-level agent/device resources during
bhnd(4) bus enumeration.

• Add a bhnd_erom_probe() method usable by bhndb(4). This is an analogue to the existing bhnd_erom_probe_static() method, and allows the bhndb bridge to discover the best available erom parser class prior to newbus probing of its children.
• Add support for supplying identification hints when probing erom devices. This is required on early EXTIF-only chipsets, where chip identification registers are not available.
• Migrate bhndb over to the new bhnd_erom API, using bhnd_core_info records rather than bridged bhnd(4) device_t references to determine the bridged chipsets' capability/bridge configuration.
• The bhndb parent (e.g. if_bwn) is now required to supply a hardware priority table to the bridge. The default table is currently sufficient for our supported devices.
• Drop the two-pass attach approach we used for compatibility with bhndb(4) in the bhnd(4) bus drivers, and instead perform bus enumeration immediately, and allocate bridged per-child bus-level resources during that enumeration.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7768