scmi_perf: Add support for pending requests

This patch adds support for handling deferred requests to
the SCMI performance module.

It follows the 'deferred_response_architecture' approach.

For SCMI, this means that when the request is deferred,
the response to the channel is delayed until the request
has been processed.

Change-Id: Ifc71f5976d0bf5f11ecff93ff7b9bb40cc208982
Signed-off-by: Jim Quigley <jim.quigley@arm.com>