Add support for querying the attachment type of the bhnd bus

[landon_landonf.org]

Summary

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.

Depends on D6491

Test Plan

Tested against BCM4331 and BCM4312 devices.