bhnd(4): Perform explicit chipc child enumeration.

Replaces use of DEVICE_IDENTIFY with explicit enumeration of chipc
child devices using the chipc capability structure.

This is a precursor to PMU support, which requires more complex resource
assignment handling than achievable with the static device name-based
hints table.

Reviewed by: Michael Zhilin <mizkha@gmail.com> (Broadcom MIPS support)
Approved by: re (gjb), adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D6896