watchdog: sp5100_tco: Add support for get_timeleft

Tested on a Gigabyte X570 I AORUS PRO WIFI.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210928065735.548966-1-linux@weissschuh.net


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>