firmware: arm_scmi: Add notification callbacks-registration

Add the core SCMI notifications callbacks-registration support: allow
users to register their own callbacks against the desired events.

Whenever a registration request is issued against a still non existent
event, mark such request as pending for later processing, in order to
account for possible late initializations of SCMI Protocols associated
to loadable drivers.

Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Jonathan Cameron <[email protected]>
Signed-off-by: Cristian Marussi <[email protected]>
Signed-off-by: Sudeep Holla <[email protected]>