watchdog: atlas7: add watchdog driver of CSRatlas7

This patch adds watchdog driver for CSRatlas7 platform.
On CSRatlas7, the 6th timer can act as a watchdog timer
when the Watchdog mode is enabled.

Signed-off-by: Guo Zeng <Guo.Zeng@csr.com>
Signed-off-by: William Wang <William.Wang@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>