firmware: arm_ffa: Implement the FFA_NOTIFICATION_GET interface

The framework provides an interface to the receiver to determine the
identity of the notification. A receiver endpoint must use the
FFA_NOTIFICATION_GET interface to retrieve its pending notifications
and handle them.

Add the support for FFA_NOTIFICATION_GET to allow the caller(receiver)
to fetch its pending notifications from other partitions in the system.

Link: https://lore.kernel.org/r/20231005-ffa_v1-1_notif-v4-6-cddd3237809c@arm.com


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