MIPS: BCM47XX: Add button and led configuration for some Linksys devices

This adds led and button GPIO configuration for Linksys wrt54g3gv2,
wrt54gsv1 and wrtsl54gs. This is based on OpenWrt broadcom-diag code.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Cc: zajec5@gmail.com
Patchwork: https://patchwork.linux-mips.org/patch/6592/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>