MIPS: BCM47xx: Extend bcm47xx_fill_sprom with prefix.

MIPS: BCM47xx: Extend bcm47xx_fill_sprom with prefix.

When an other SSB based device without an own SPROM is attached, using the
PCI bus to the main SSB based device, the data normally found in the SPROM
will be stored in the NVRAM on modern devices. The keys, to load the data
from the NVRAM, are all using some sort of prefix like pci/1/1/, pci/1/3/
or sb/1/ before the actual key. This patch extends bcm47xx_fill_sprom() to
make it possible to read out these values when some prefix was used.

The keys for the SPROM data used on the main chip does not have a prefix.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2363/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>