dt-bindings: uio: Add msm_sharedmem driver bindings

Add bindings for msm_sharedmem driver. This driver will
allocate the sharedmem region and assign the access
permission to the shared region.

Change-Id: Id60e1cf7afbe25912f82a0e202c2dce98ceddeec
Signed-off-by: Ankit Jain <jankit@codeaurora.org>