brcm47xx: add buttons support for Netgear WNR3500L

Ticket: #14406

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41547 3c298f89-4303-0410-b956-a3cf2f4a3e73