feat(rdn2): add board support for rdn2cfg2 variant

Add board support for variant 2 of RD-N2 platform which is a four chip
variant with 4 cores on each chip. The "CSS_SGI_PLATFORM_VARIANT" value
is 2 for multi-chip variant. The "CSS_SGI_CHIP_COUNT_MACRO" can be in
the range [1, 4] for multi-chip variant.

Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Change-Id: I6412106e80e2f17704c796226c2ee9fe808705ba