[bwn] [bhnd] initial support for using bhnd for if_bwn devices.

This is an initial work in progress to use the replacement bhnd
bus code for devices which support it.

• Add manpage updates for bhnd, bhndb, siba
• Add kernel options for bhnd, bhndbus, etc
• Add initial support in if_bwn_pci / if_bwn_mac for using bhnd as the bus transport for suppoted NICs
• if_bwn_pci will eventually be the PCI bus glue to interface to bwn, which will use the right backend bus to attach to, versus direct nexus/bhnd attachments (as found in embedded broadcom devices.)

The PCI glue defaults to probing at a lower level than the bwn glue,
so bwn should still attach as per normal without a boot time tunable set.

It's also not fully fleshed out - the bwn probe/attach code needs to be
broken out into platform and bus specific things (just like ath, ath_pci,
ath_ahb) before we can shift the driver over to using this.

Tested:

• BCM4311, STA mode
• BCM4312, STA mode

Submitted by: Landon Fuller <landonf@landonf.org>
Differential Revision: https://reviews.freebsd.org/D6191