driver: watchdog: reset watchdog in designware_wdt_stop() function

In uboot command line environment, watchdog is not able to be
stopped with below commands:
SOCFPGA_STRATIX10 # wdt dev watchdog@ffd00200
SOCFPGA_STRATIX10 # wdt stop
Refer to watchdog driver in linux kernel, it is also need to reset
watchdog after disable it so that the disable action takes effect.

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>