kernel: adm6996: add support for ADM6996L and GPIO interface

kernel: adm6996: add support for ADM6996L and GPIO interface

This patch makes it possible to use adm6996.c on first generation
BCM47XX devices with ADM switches.
The GPIO bit banging protocol implementation was copied from the old
switch driver and adapted to this driver and changed to the mainline
kernel GPIO interface.
The ADM6996L is different from the ADM6996M which is supported, for
both specs are available in the Internet.

This was tested on a WRT54GS version 1.0, thank you Dirk Neukirchen for
the device.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38698 3c298f89-4303-0410-b956-a3cf2f4a3e73