firmware: arm_ffa: Add schedule receiver callback mechanism

Enable client drivers to register a callback function that will be
called when one or more notifications are pending for a target
partition as part of schedule receiver interrupt handling.

Link: https://lore.kernel.org/r/20231005-ffa_v1-1_notif-v4-9-cddd3237809c@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>