brcm63xx: probe gpio controllers through DT

Add a generic mmio gpio controller based driver and probe it
through device tree.

Use aliases for base calculation until we converted all users to
device tree or named gpios.

Convert bcm63xx_enet's ephy-reset gpio to use a named gpio.

While at it, remove the duplicate reset gpio defintion for livebox.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44565 3c298f89-4303-0410-b956-a3cf2f4a3e73