rpmsg: Introduce __rpmsg{16|32|64} types

Introduce __rpmsg{16|32|64} types along with byte order conversion
functions based on an rpmsg_device operation as a foundation to
make RPMSG modular and transport agnostic.

Tested-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Suggested-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/20201120214245.172963-2-mathieu.poirier@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>