[bhnd] Add support for querying the attachment type of the bhnd bus.

This adds a BHND_BUS_GET_ATTACH_TYPE(); the primary use-case is to let
chipc make a coarse-grained determination as to whether UART, SPI, etc
drivers ought to be attached, and on fullmac devices, whether a real
CPU driver ought to be skipped for the ARM core, etc.

Tested:

• BCM4331 (BHND)
• BCM4312 (SIBA)

Submitted by: Landon Fuller <landonf@landonf.org>
Differential Revision: https://reviews.freebsd.org/D6492