gpio-button-hotplug: add wwan button

The wimax key will be used as a generic wwan key starting with Linux
3.13. The brcm47xx target uses this key for the 3g buttons. Also remove
the ifdef around KEY_WPS_BUTTON, this is in the kernel for a long time
now.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 39290