tee: replace cdev_add + device_add with cdev_device_add

Commit 233ed09d ("chardev: add helper function to register char devs
with a struct device") added a helper function 'cdev_device_add'.

Make use of cdev_device_add in tee_device_register to replace cdev_add
and device_add. Since cdev_device_add takes care of setting the
kobj->parent, drop explicit initialisation in tee_device_alloc.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>