feat(plat/sgi): introduce platform variant build option

A Neoverse reference design platform can have two or more variants that
differ in core count, cluster count or other peripherals. To allow reuse
of platform code across all the variants of a platform, introduce build
option CSS_SGI_PLATFORM_VARIANT for Arm Neoverse reference design
platforms. The range of allowed values for the build option is platform
specific. The recommended range is an interval of non negative integers.

An example usage of the build option is

Change-Id: Iaae79c0b4d0dc700521bf6e9b4979339eafe0359
Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
8 files changed