rpmsg: Turn name service into a stand alone driver

Make the RPMSG name service announcement a stand alone driver so that it
can be reused by other subsystems. It is also the first step in making the
functionatlity transport independent, i.e that is not tied to virtIO.

Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Tested-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Co-developed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Co-developed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Link: https://lore.kernel.org/r/20201120214245.172963-9-mathieu.poirier@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

rpmsg: Move structure rpmsg_ns_msg to header file

Move structure rpmsg_ns_msg to its own header file so that
it can be used by other entities.

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/20201120214245.172963-4-mathieu.poirier@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>