tee: export teedev_open() and teedev_close_context()

Exports the two functions teedev_open() and teedev_close_context() in
order to make it easier to create a driver internal struct tee_context.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>