bhnd(4): Implement a new bhnd_nvram_plist and bhnd_nvram_prop API for
representing arbitrary Broadcom NVRAM key/value pairs.

This will be used to track pending changes in bhnd_nvram_store, and
provide support for exporting all or a device subpath for NVRAM (as
required by some fullmac wifi chipsets).

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D8756