refactor(sgi): replace references to "SGI"/"sgi" for neoverse_rd

Currently, there are several reference to "SGI" or "sgi" in comments or
as macro prefix within the neoverse_rd directory. As part of the
migration to neoverse_rd, rename all occurences of SGI/sgi to
Neoverse-RD or the NRD prefix accordingly. All references in comments
are rephrased as "Neoverse RD platforms". References in code are renamed
as "NRD"/"nrd" accordingly.

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: I55bdab01051974b3ddb2dae3bc4c0e253fb85fc3