sandbox: fix sandbox_wdt_expire_now()

With CONFIG_SYSRESET_WATCHDOG=y the sandbox can use a watchdog based system
reset.

To make this work calling sandbox_wdt_expire_now() must lead to a reset.

With this change we can test the development suggested in

  [PATCH 0/4] Improved sysreset/watchdog uclass integration
  https://lists.denx.de/pipermail/u-boot/2021-August/458656.html



Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>