scp_v2.11: Release notes for SCP-firmware v2.11

With this patch the change log is updated to contain
description for new features, improvements and fixes contained
in this release.

Please note that some remarks have been added for the new
added tool Unit Test and its implications for the contributors.

Change-Id: Iac4a3bcef51d818dbfa88207d5fb857eeea0c4e7
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>