Introduce Retu watchdog driver.

Cc: linux-watchdog@vger.kernel.org
Acked-by: NFelipe Balbi <balbi@ti.com>
Acked-by: NTony Lindgren <tony@atomide.com>
Signed-off-by: NAaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>