Complete the virtio_rpmsg_channel structure description to fix a
compilation warning with W=1 option:

drivers/rpmsg/virtio_rpmsg_bus.c:95: warning: Cannot understand
 * @vrp: the remote processor this channel belongs to
Reviewed-by: NMathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: NArnaud Pouliquen <arnaud.pouliquen@st.com>
Link: https://lore.kernel.org/r/20200731074850.3262-1-arnaud.pouliquen@st.comSigned-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>