[WATCHDOG] AR7: watchdog timer

Driver for the watchdog timer. Still doesn't reboots the machine
on some boards, but we have improved and cleaned it

Signed-off-by: Matteo Croce <technoboy85@gmail.com>
Signed-off-by: Nicolas Thill <nico@openwrt.org>
Signed-off-by: Enrik Berkhan <Enrik.Berkhan@akk.org>
Signed-off-by: Christer Weinigel <wingel@nano-system.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>