firmware: arm_ffa: Add support for {un,}registration of framework notifications

Framework notifications are doorbells that are rung by the partition
managers to signal common events to an endpoint. These doorbells cannot
be rung by an endpoint directly. A partition manager can signal a
Framework notification in response to an FF-A ABI invocation by an
endpoint.

Two additional notify_ops interface is being added for any FF-A device/
driver to register and unregister for such a framework notifications.

Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Message-Id: <20250217-ffa_updates-v3-16-bd1d9de615e7@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>