MIPS: bcm47xx: read baordrev without prefix from sprom

When the boardrev with a prefix is not available, try to read it
without a prefix. This is based on code from the Broadcom SDK.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>