bhnd(4): Implement generic support for discovery of platform devices (PMU, NVRAM, ChipCommon, etc).

[landonf]

Summary

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.

Test Plan

Verified that the NVRAM device (if supported) is still discovered, and
that NVRAM lookups succeed, e.g.:

bwn0: got macaddr 68:a3:c4:25:fa:5a

Tested on:

• BCM4312 (siba, bhndb-attached nvram)
• BCM43225 (bcma, chipc-attached nvram)
• RT-N53 (mips/bcma, nvram support unimplemented)

[landonf]

Treat bhnd nvram/pmu/chipc devices as valid if they are currently attaching, rather than only after attachment is complete.

[adrian]

as long as it survives make universe, approved!

[landonf]

Fixed an embarassing copy/paste typo helpfully caught by GCC when testing the universe build.