watchdog: davinci_wdt: add restart function

This adds a restart function to the davinci watchdog timer driver.

This is copied from arch/arm/mach-davinci/time.c and will allow us to
remove the code from there.

Signed-off-by: David Lechner <david@lechnology.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>