MIPS: BCM47xx: read out full board data

Read out the full board data independently of the sprom version. Now we
also get the full boardflags and so on if sromrev is not set and our
code would assume a rev 1 device. When a nvram option is not set
because it is not there this is no problem.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4363
Signed-off-by: John Crispin <blogic@openwrt.org>