watchdog: Add Spreadtrum watchdog driver

This patch adds the watchdog driver for Spreadtrum SC9860 platform.

Signed-off-by: Eric Long <eric.long@spreadtrum.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>