watchdog: sirf: add watchdog driver of CSR SiRFprimaII and SiRFatlasVI

On CSR SiRFprimaII and SiRFatlasVI, the 6th timer can act as a watchdog
timer when the Watchdog mode is enabled.

watchdog occur when TIMER watchdog counter matches the value software
pre-set, when this event occurs, the effect is the same as the system
software reset.

Signed-off-by: Xianglong Du <Xianglong.Du@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Cc: Romain Izard <romain.izard.pro@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>