firmware: arm_ffa: Add notification handling mechanism

With all the necessary plumbing in place, let us add handling the
notifications as part of schedule receiver interrupt handler. In order
to do so, we need to just register scheduling callback on behalf of the
driver partition.

Link: https://lore.kernel.org/r/20231005-ffa_v1-1_notif-v4-12-cddd3237809c@arm.com


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