sample: add OpenAMP sample application

This commit adds a sample application using OpenAMP for remote procedure
calls on the LPCXpresso54114. It is adapted from the RPMsg-Lite sample
application added in PR #5960, and uses the IPM driver to provide
interprocessor interrupts.

Signed-off-by: Kristian Klomsten Skordal <kristian.skordal@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>