BACKPORT: tee: use reference counting for tee_context

We need to ensure that tee_context is present until last
shared buffer will be freed.

Change-Id: I0f16c080e0432d425496a7ceb11d13150daff00e
Signed-off-by: Volodymyr Babchuk <vlad.babchuk@gmail.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
(cherry picked from commit 217e0250cccb9e54d457991446cd3fab413085e1)
Signed-off-by: Victor Chong <victor.chong@linaro.org>