tee: use reference counting for tee_context

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

Signed-off-by: Volodymyr Babchuk <vlad.babchuk@gmail.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

drivers/tee/tee_private.h

@@ -73,4 +73,7 @@ int tee_shm_get_fd(struct tee_shm *shm);
 bool tee_device_get(struct tee_device *teedev);
 void tee_device_put(struct tee_device *teedev);
 
+void teedev_ctx_get(struct tee_context *ctx);
+void teedev_ctx_put(struct tee_context *ctx);
+
 #endif /*TEE_PRIVATE_H*/