dt-bindings: Document the mem-buf capabilities property

Add documentation for the "qcom,mem-buf-capabilities" dt-binding
for the mem-buf device. This property defines the role of an
instance of a mem-buf driver on a particular Virtual Machine (VM)
as per the supplier and consumer model that the driver follows.

Change-Id: Ic20476fb5aa5e21f8b78e19a47b4f682e35e6a36