refactor(rdn2): reduce use of CSS_SGI_PLATFORM_VARIANT build flag

refactor(rdn2): reduce use of CSS_SGI_PLATFORM_VARIANT build flag

The core count is one of the significant difference between the various
RD-N2 platform variants. The PLAT_ARM_CLUSTER_COUNT macro defines the
number of core/cluster for a variant. In preparation to add another
variant of RD-N2 platform, replace the use of CSS_SGI_PLATFORM_VARIANT
build flag, where applicable, with the PLAT_ARM_CLUSTER_COUNT macro.
This helps to reduce the changes required to add support for a new
variant.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Change-Id: I89b168308d1b5f7edd402205dd25d6c3a355e100