BACKPORT: tee: shm: add page accessor functions

In order to register a shared buffer in TEE, we need accessor
function that return list of pages for that buffer.

Change-Id: I34630ae5d69813639ac8f0988631a65c16432846
Signed-off-by: Volodymyr Babchuk <vlad.babchuk@gmail.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
(cherry picked from commit e0c69ae8bfb500facebe1fa331f9400c216eaab0)
Signed-off-by: Victor Chong <victor.chong@linaro.org>