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" and the source files have been migrated out of
the css directory, replace the prefix "CSS_SGI" with "NRD".

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: I27989ff42404d823dd2a8cd22ff485497ccb62d4
22 files changed