ipc_service: Add new RPMsg with static VRINGs backend

Introduce a new RPMsg with static VRINGs backend. This new backend makes
easy to generate and use IPC instances backed by OpenAMP using the DT.

Each instance is defined in the DT as (for example):

  ipc: ipc {
          compatible = "zephyr,ipc-openamp-static-vrings";
          shm = <&sram_ipc0>;
          mboxes = <&mbox 0>, <&mbox 1>;
          mbox-names = "tx", "rx";
          role = "primary";
          status = "okay";
  };

It is then possible to register an send data through endpoints using the
IPC service APIs.

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