refactor(sgi): replace build-option prefix with "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: I3ef01fedd6d955631bf39a3bf31c741cdeadf1cc
7 files changed