platform/rdn1edgex2: add build configs for rdn1edge dual chip platform

RdN1Edge platform supports a dual-chip configuration wherein two
rdn1edge SoCs are connected through a high speed coherent CCIX link.
Add support for building rdn1edge dual chip platform. This dual-chip
RDN1Edge platform name is rdn1edgex2 and should be used as the value
of the parameter '-p' during build.

Change-Id: Ide2709aa7a6dac21b4eb5fbf0f69806f1f5b3464
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>