Platform/Sgi: Add support for RD-N2-Cfg3 platform

The Neoverse RD-N2-Cfg3 platform is a variant of RD-N2 platform with a
different mesh size and GIC ITS count. As part of the initial platform
support, add the corresponding platform and flash description files.

A new PCD named PcdGicItsCount is introduced to specify the number of
GIC ITS blocks supported by the platform. The default value of this PCD
is set to 6 which is number of ITS blocks supported by the existing
RD-N2 variant platforms. For RD-N2-Cfg3, this PCD is set to a value of
12.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Change-Id: I80926f19a175f8b84e9411556ebb5ebcc45a38fc