Per System Control Management Interface specification:
"Completion interrupts: This transport supports polling or interrupt driven
 modes of communication. In interrupt mode, when the callee completes
 processing a message, it raises an interrupt to the caller. Hardware
 support for completion interrupts is optional."

So, add an optional mailbox channel for notification completion interrupts.
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20240510-scmi-notify-v2-1-e994cf14ef86@nxp.comSigned-off-by: Sudeep Holla <sudeep.holla@arm.com>