Add a reference link to FVP RDN2 PoC README since
FVP RDV3R1 model isn't available yet.

Signed-off-by: John Chung <john.chung@arm.com>
Change-Id: If2cf68a62f61a5a7953c1ed7c37653d06425c336

FVP PoC includes in-band and side-band interface
between BMC and Arm SoC (AP and MCP) via serial.

* PLDM Sensor Reading
* PLDM CPER Event
* IPMI In-Band

Signed-off-by: John Chung <john.chung@arm.com>
Change-Id: Ia17e922b9c23064354e6405888b9eb76b66fab93