rpmsg: Make rpmsg_{register|unregister}_device() public

Make function rpmsg_register_device() and rpmsg_unregister_device()
functions public so that they can be used by other clients.  While
doing so get rid of two obsolete function, i.e register_rpmsg_device()
and unregister_rpmsg_device(), to prevent confusion.

Tested-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Link: https://lore.kernel.org/r/20201120214245.172963-8-mathieu.poirier@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>