infra/common: add documentation for MCP sideband channel

Add guidelines for enabling MCP sideband channel on Neoverse Reference
Design Platfroms. In addition to enablement, add steps for parsing
ouput logs and understanding the basic design behind the implementation.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: I0e8b217d0a2890d6c2be94b4bc6e0571f26ab5ba