watchdog: sp805_wdt: add expire_now method

Add sp805_wdt_expire_now function.
expire_now method is required by U_BOOT_DRIVER.

Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>
Signed-off-by: Zhao Qiang <qiang.zhao@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>