refactor(sgi): rename CSS_SGI macro prefixes to NRD

As of now, several internal macros utilize "CSS_SGI" as their prefix.
Given the change to neoverse_rd, the prefix "CSS_SGI" is no longer
appropriate. Therefore, update the macro prefixes to "NRD" for
consistency and clarity.

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: I252a5ece6c82a0a5a3b451238ed0da07f8a0c81e