bhnd(4): extend the PMU APIs to support bwn(4)

The bwn(4) driver requires a number of extensions to the bhnd(4) PMU
interface to support external configuration of PLLs, LDOs, and other
parameters that require chipset or PHY-specific workarounds.

These changes add support for:

- Writing raw voltage register values to PHY-specific LDO regulator
  registers (required by LP-PHY).
- Enabling/disabling PHY-specific LDOs (required by LP-PHY)
- Writing to arbitrary PMU chipctrl registers (required for common PHY PLL
  reset support).
- Requesting chipset/PLL-specific spurious signal avoidance modes.
- Querying clock frequency and latency.

Additionally, rather than updating legacy PWRCTL support to conform to the
new PMU interface:

- PWRCTL API is now provided by a bhnd_pwrctl_if.m interface.
- Since PWRCTL is only found in older SSB-based chipsets, translation from
  bhnd(4) bus APIs to corresponding PWRCTL operations is now handled
  entirely within the siba(4) driver.
- The PWRCTL-specific host bridge clock gating APIs in bhnd_bus_if.m have
  been lifted out into a standalone bhnd_pwrctl_hostb_if.m interface.

Approved by:	adrian (mentor, implicit)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D12664

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