MIPS: BCM47XX: remove GPIO driver

Instated of providing an own GPIO driver use the one provided by ssb and
bcma.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4592


Acked-by: Florian Fainelli <florian@openwrt.org>