BACKPORT: tee: add start argument to shm_register callback

Adds a start argument to the shm_register callback to allow the callback
to check memory type of the passed pages.

Change-Id: Ic6062db0cfc7485adae4df949e87577f9d13e4d5
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
(cherry picked from commit 95ffe4ca43877eea176d7e95aa0d38bbdc3d2903)
Signed-off-by: Victor Chong <victor.chong@linaro.org>