samples: openamp: Update sample to OpenAMP v2018.10 release

The v2018.10 release of OpenAMP reworks the API set and splits the
remoteproc vs rpmsg interfaces so one can use rpmsg without remoteproc.

This helps drastically reduce the code footprint utilized by OpenAMP.
The remote see around 4k reduction in code size.

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