watchdog: f71808e_wdt: new watchdog driver for Fintek F71808E and F71882FG

Add a new watchdog driver for the Fintek F71808E and F71882FG Super I/O
controllers.

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>