meta-arm: Add SCP MCP build support

SCP-firmware provides a software reference implementation for
the System Control Processor (SCP) and Manageability Control
Processor (MCP) components found in several Arm Compute
Sub-Systems.

This patch set adds support to fetch and build SCP and MCP
firmware binaries

Issue-Id: PLATFORMS-3134

Change-Id: Ic7259bb430c9e7e9711c9c8bc3283aafaacdf707
Signed-off-by: Patrik Berglund <patrik.berglund@arm.com>
Signed-off-by: Khasim Syed Mohammed <khasim.mohammed@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>