ipc: ipc_service: Rework multi-instance backend.

Only one single IPC service backend is currently present: multi_instance
backend. This backend is heavily relying on the RPMsg multi_instance
code to instanciate and manage instances and endpoints. Samples exist
for both in the samples/subsys/ipc/ directory.

With this patch we are "unpacking" the RPMsg multi_service code to make
it more modular and reusable by different backends.

In particular we are re-organizing the code into two helper libraries:
an RPMsg library and a VRING / virtqueues static allocation library. At
the same time we rewrite the multi_instance backend to make fully use of
those new libraries and remove the old multi_instance sample.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>