dt-bindings: Document the qcom,vmid-cp-camera-preview-ro property

Document the qcom,vmid-cp-camera-preview-ro secure buffer property.
This property is used to communicate to the secure buffer driver that
buffers accessible to VMID_CP_CAMERA_PREVIEW should be mapped with
read-only permissions.

Change-Id: Ib867fd2a060448fc6996fddde6367fa4e6c1d5d8