bhnd(4): support IPX OTP NVRAM/SPROM data source

Add support for fetching SPROM data from OTP on chipsets with an
IPX OTP controller (including the BCM43225).

This integrates the NVRAM data source into the chipc_caps capability
structure, and adds a sprom_offset field that can be used with OTP
to locate the SPROM image data (found within the general use
region, H/W subregion).

This also removes one of two duplicate parse error messages reported by
both the bhnd_sprom driver and the underlying SPROM parsing API.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D6729