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
