MAINTAINERS: add entry for HPE watchdog driver

Add a maintainer for the HPE watchdog driver

Signed-off-by: Brian Boylston <brian.boylston@hpe.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>