tee: add register user memory

Added new ioctl to allow users register own buffers as a shared memory.

Signed-off-by: Volodymyr Babchuk <vlad.babchuk@gmail.com>
[jw: moved tee_shm_is_registered() declaration]
[jw: added space after __tee_shm_alloc() implementation]
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>