watchdog: acquirewdt: Use platform_driver_probe

Using platform_driver_probe instead of platform_driver_register has
two benefits:
* The driver will fail to load if device probing fails.
* The probe function can be marked __init.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>