MIPS: BCM47xx: Distinguish WRT54G series devices by boardtype

Catalin reported that GPIOs used by bcm47xx don't match layout of his
WRT54GS V1.0 board. It seems we need to distinguish these 54G* devices.

Reported-by: Catalin Patulea <cat@vv.carleton.ca>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: https://patchwork.linux-mips.org/patch/7112/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>