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).

I also cleaned up a case of duplicate error messages reported by both
the bhnd_sprom driver and the underlying SPROM parsing API.

OTP support is currently integrated into the existing chipc SPROM driver;
if we add SPROM-specific NVRAM write support in the future, we can separate
this out into distinct SPROM and OTP drivers.

Depends on D6712

Tested OTP handling against BCM43225:

bwn_pci0: <Broadcom BCM43225 802.11n 2GHz Wireless> mem 0xc0004000-0xc0007fff irq 17 at device 4.0 on pci0
...
bwn0: <802.11 MAC/PHY/Radio> mem 0x18001000-0x18001fff,0x18101000-0x18101fff at core 1 on bhnd0
bwn0: activated 0x18001000-0x18001fff as direct resource
bwn0: got rid=0 res=0xfffff80003705ee0
bwn0: got macaddr 68:a3:c4:25:fa:5a

Tested for regressions in SPROM/flash handling against:

• BCM4331
• BCM4312
• RT-N16
• WGT634U