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.
Use PcdPlatformVariant for the RD-N2-Cfg3 platform to specify the
platform variant. RD-N2-Cfg3 has 12 GIC ITS blocks when compared to the
other RD-N2 variants that have 6 GIC ITS blocks.

Signed-off-by: Prabin CA <prabin.ca@arm.com>