ipc: ipc_service: Add support for multiple instances

The IPC service code is currently assuming that only one IPC instance
does exist and the user can use the IPC service API to interface with
that singleton instance.

This is a huge limitation and this patch is trying to fix this
assumption introducing three major changes to the IPC service API:

- All the IPC instances are now supposed to be instantiated as a struct
  device. A new test is introduced to be used as skeleton for all the
  other backends.

- ipc_service_register_backend() is now removed (because multiple
  backends are now supported at the same time).

- All the other ipc_service_*() functions are now taking a struct device
  pointer as parameter to specify on which instance the user is going to
  act and operate.

In this patch the documentation is also extended to better clarify the
terminology used.

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