refactor(sgi): replace build-option prefix to NRD

As of now, CSS_SGI_PLATFORM_VARIANT and CSS_SGI_CHIP_COUNT are the
external build option that sgi platforms support. As sgi has been
renamed to neoverse_rd, replace the prefix CSS_SGI with NRD.

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: I585575788c14d1c3b26ab336672ef149771dab98