MAINTAINERS: Add Guenter Roeck as co-maintainer of watchdog subsystem

Having a co-maintainer will enable us to be more flexible with
pull requests.

Cc: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>