ar71xx: add support for WIFI button on tp-link tl-mr3220v2

The TP-LINK TL-MR3220 v2 has a button on the left side
labeled "WIFI". This is GPIO 24. This patch adds the
support for this button.

Signed-off-by: Vittorio Gambaletta <openwrt@vittgam.net>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36848 3c298f89-4303-0410-b956-a3cf2f4a3e73