MIPS: BCM47XX: Prepare support for GPIO buttons

So far this adds support for one Netgear model only, but it's designed
and ready to add many more device. We could hopefully import database
from OpenWrt.
Support for SSB is currently disabled, because SSB doesn't implement IRQ
domain yet.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6300/