ipc: RPMsg service to register multiple endpoints

This patch implements a service that adds multiendpoint
capabilities to RPMsg. Multiple endpoints are intended to be used
when multiple modules need services from a remote processor. Each
module may register one or more RPMsg endpoints.

The implementation separates backend from the service, what
allows to extend this module to support other topologies like
Linux <-> Zephyr.

Co-authored-by: Piotr Szkotak <piotr.szkotak@nordicsemi.no>
Signed-off-by: Hubert Miś <hubert.mis@nordicsemi.no>