watchdog: it87_wdt: add IT8783 ID

IT8783 watchdog works as in IT872x

Tested on Adlink cPCI-6520 boards

Signed-off-by: Paolo Teti <paolo.teti@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>