watchdog: hpwdt: Add support for WDIOC_SETOPTIONS

WDIOC_SETOPTIONS makes it possible to disable and re-enable the
watchdog timer while the hpwdt driver is loaded.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>