ext: open-amp: Update OpenAMP to v2018.10 release

Update to new OpenAMP v2018.10 release.  This release allows us to
utilize just rpmsg without remoteproc.  The API set has changed and
requires updates to the openamp sample.  Additionally, the changes
in this release reduce the code size footprint, and support a static
allocation memory model.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>