dt-bindings: uio: msm_sharedmem: Add DT binding entry for guard-memory

Add DT binding entry guard-memory to add the guard area by SZ_4K at the
start and at the end of the shared mem region.

Change-Id: If86b8bdf68abea5e6fd3761aa6bbadfe4e8ea874
Signed-off-by: Ankit Jain <jankit@codeaurora.org>