platform/rdn2: add PCIe hierarchy with PCIe Exerciser instantiated

Add an example PCIe hierarchy that allows instantiation of PCIe
Exerciser to enable various tests around it. The json file reprsenting
this hierarchy is placed as an example and it is not connected to the
root port for the tests. For using this hierarchy, the platforms
run_model.sh file will have to be updated.

Suggested-by: Gowtham Siddarth <gowtham.siddarth@arm.com>
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>