BACKPORT: tee: shm: add accessors for buffer size and page offset

These two function will be needed for shared memory registration in OP-TEE

Change-Id: If6a16ec82a2880c05a37c05bf00e5722b11efc64
Signed-off-by: Volodymyr Babchuk <vlad.babchuk@gmail.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
(cherry picked from commit b25946ad951c013c31d0a0e82d2017004bdc8fed)
Signed-off-by: Victor Chong <victor.chong@linaro.org>