bhnd(4): Add support for querying DMA address translation parameters

BHND Wi-Fi chipsets and SoCs share a common DMA engine, operating within
backplane address space. To support host DMA on Wi-Fi chipsets, the bridge
core maps host address space onto the backplane; any host addresses must
be translated to their corresponding backplane address.

• Defines a new bhnd_get_dma_translation(9) API to support querying DMA address translation parameters from the bhnd(4) bus.
• Extends bhndb(4) to provide DMA translation descriptors from a DMA address translation table defined in the host bridge-specific bhndb_hwcfg.
• Defines bhndb(4) DMA address translation tables for all supported host bridge cores.
• Extends mips/broadcom's bhnd_nexus driver to return an identity (no-op) DMA translation descriptor; no translation is required when addressing the SoC backplane.

Approved by: adrian (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12582