ipc_service: Extend RPMsg structs and misc fixes

Extend the RPMsg structs to accommodate for the introduction of new
backends and contextually fix the ipc_rpmsg_static_vrings_mi backend
(the only user).

Rework also some comments and ipc_service glue code.

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