tee: replace tee_shm_register()

tee_shm_register() is replaced by the previously introduced functions
tee_shm_register_user_buf() and tee_shm_register_kernel_buf().

Since there are not external callers left we can remove tee_shm_register()
and refactor the remains.

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