base-files: implemented basic GPIO control

Internal GPIO pins are used for PoE passthrough setups in multi-port
routers. This patch implemnets control over this hardware feature for
Ubiquiti Nanostations and TP-Link CPE510.

Signed-off-by: Lars Kruse <lists@sumpfralle.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46271 3c298f89-4303-0410-b956-a3cf2f4a3e73