MIPS: BCM47XX: use fallback sprom var for board_{rev,type}

An SoC normally do not define path variables for board_rev and
board_type and the Broadcom SDK also uses the nvram values without a
prefix in such cases. Do the same to fill these sprom attributes from
nvram and do not leave them empty, because brcmsmac do not like this.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4679/
Signed-off-by: John Crispin <blogic@openwrt.org>