MIPS: bcm47xx: refactor fetching board data

Now the fetching of board data also uses nvram_read_u16 and not
simple_strtoul any more.

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>