Input: define KEY_WWAN for Wireless WAN

Some devices with support for mobile networks may have buttons for
enabling/disabling such connection. An example can be Linksys router 54G3G.
We already have KEY_BLUETOOTH, KEY_WLAN and KEY_UWB so it makes sense to
add KEY_WWAN as well.  As we already have KEY_WIMAX, use it's value for
KEY_WWAN and make it an alias.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>