arm-bsp/docmumentation: Add information about tc0

This includes an overview as well as build and run instructions
for the tc0 software stack.

Change-Id: I9abfc7d6dffc68da2a2cd4ceda82bec6bc5fcb10
Signed-off-by: Usama Arif <usama.arif@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>