bhnd(4): Add bus pass-aware discovery of platform devices (PMU,
NVRAM, ChipCommon, etc).

This extends the existing handling of NVRAM core discovery to support
locating additional devices that may be attached either directly as real
cores, or indirectly via ChipCommon (e.g. bhnd_pmu).

When attached as a SoC root bus (as opposed to a bridged WiFi device),
the platform devices may not be attached until later bus passes,
necessitating delayed discovery/initialization.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D6962