MIPS: BCM47XX: move and extend sprom parsing

Move the sprom parsing from nvram into sprom.c. There are all values
needed for sprom version 1 to 9 read from nvram and there are more
sanity checks added. This is based on the sprom parsing in the open
source part of the Broadcom SDK.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>