bhnd(4): Initial PMU/PWRCTL power and clock management support.
- Added bhnd_pmu driver implementations for PMU and PWRCTL chipsets, derived from Broadcom's ISC-licensed HND code.
- Added bhnd bus-level support for routing per-core clock and resource power requests to the PMU device.
- Lift ChipCommon support out into the bhnd module, dropping bhnd_chipc.
Reviewed by: mizhka
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7492