Add documentation for the console framework on how to go about
instantiating a new console and how to use these consoles in TF-A.
This includes BOOT, RUNTIME and CRASH consoles.
Change-Id: I746d38f69f1b035d2e85d2589646e7fd67cb9cc3
Signed-off-by: Salman Nabi <salman.nabi@arm.com>