brcm47xx: add fallback sprom for pci devices without an own sprom.

brcm47xx: add fallback sprom for pci devices without an own sprom.

If there is no sprom on an ssb based pci device on the brcm47xx
architecture ssb now asks the architecture code to look into the nvram
to get some sprom data for this device. Now we are able to read out
pci/1/1/ foo or pci/1/3/ foo config options.

This will fix some problems where the wireless devices does not got an
mac address and the following message was show:
ssb: WARNING: Invalid SPROM CRC (corrupt SPROM)

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@26801 3c298f89-4303-0410-b956-a3cf2f4a3e73