rtc: optee: Migrate to use tee specific driver registration function

rtc: optee: Migrate to use tee specific driver registration function

The tee subsystem recently got a set of dedicated functions to register
(and unregister) a tee driver. Make use of them. These care for setting the
driver's bus (so the explicit assignment can be dropped) and the driver
owner (which is an improvement this driver benefits from).

Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>