watchdog: meson_wdt: fix register and callback of reboot notfiy

watchdog: meson_wdt: fix register and callback of reboot notfiy

PD#147089: watchdog: meson_wdt: fix register and callback of reboot notify

The function register_reboot_notifier() use the wrong notifier_block
variable. So the container_of in the callback can't get the right pointer
of the struct.

The watchdog is disable in case of system down. If the kernel panic, the
reboot notify should ignore that.

Change-Id: I3fd21590808f4b380ddd54883db576e5c13c04a1
Signed-off-by: Bo Yang <bo.yang@amlogic.com>