Introduce bwn(4) support for the bhnd(4) bus.

Currently, bwn(4) relies on the siba_bwn(4) bus driver to provide support
for the on-chip SSB interconnect found in Broadcom's older PCI(e) Wi-Fi
adapters. Non-PCI Wi-Fi adapters, as well as the newer BCMA interconnect
found in post-2009 Broadcom Wi-Fi hardware, are not supported by
siba_bwn(4).

The bhnd(4) bus driver (also used by the FreeBSD/MIPS Broadcom port)
provides a unified kernel interface to a superset of the hardware supported
by siba_bwn; by attaching bwn(4) via bhnd(4), we can support both modern
PCI(e) Wi-Fi devices based on the BCMA backplane interconnect, as well as
Broadcom MIPS WiSoCs that include a D11 MAC core directly attached to their
SSB or BCMA backplane.

This diff introduces opt-in bwn(4) support for bhnd(4) by providing:

• A small bwn(4) driver subclass, if_bwn_bhnd, that attaches via bhnd(4) instead of siba_bwn(4).
• A bhndb(4)-based PCI host bridge driver, if_bwn_pci, that optionally probes at a higher priority than the siba_bwn(4) PCI driver.
• A set of compatibility shims that perform translation of bwn(4)'s siba_bwn function calls into their bhnd(9) API equivalents when bwn(4) is attached via a bhnd(4) bus parent. When bwn(4) is attached via siba_bwn(4), all siba_bwn function calls are simply passed through to their original implementations.

To test bwn(4) with bhnd(4), place the following lines in loader.conf(5):

hw.bwn_pci.preferred="1"

if_bwn_pci_load="YES
bwn_v4_ucode_load="YES"
bwn_v4_lp_ucode_load="YES"

To verify that bwn(4) is using bhnd(4), you can check dmesg:

bwn0: <Broadcom 802.11 MAC/PHY/Radio, rev 15> ... on bhnd0

... or devinfo(8):

pcib2

pci2
  bwn_pci0
    bhndb0
      bhnd0
        bwn0
        ...

bwn(4)/bhnd(4) has been tested for regressions with most chipsets currently
supported by bwn(4), including:

• BCM4312
• BCM4318
• BCM4321

With minimal changes to the DMA code (not included in this commit), I was
also able to test support for newer BCMA devices by bringing up basic
working Wi-Fi on two previously unsupported, BCMA-based N-PHY chipsets:

• BCM43224
• BCM43225

Approved by: adrian (mentor, implicit)
Sponsored by: The FreeBSD Foundation & Plausible Labs
Differential Revision: https://reviews.freebsd.org/D13041