pds_vdpa: pds_vdps.rst and Kconfig

Add the documentation and Kconfig entry for pds_vdpa driver.

Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230519215632.12343-12-shannon.nelson@amd.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>