scmi_power_domain: Add support for debug pending requests

This patch adds support for pending requests to the
debug HAL.

It follows the 'deferred_response_architecture' approach.

Change-Id: Idb941d028d6c267b36861e8b8ba7ac6d37f6ffe5
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Signed-off-by: Raphael Gault <raphael.gault@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>