refactor(sgi): move from "sgi" to "neoverse_rd"

Currently, reference design platforms such as RD-N2, RD-N1-Edge,
RD-V1-MC, RD-V1 and SGI-575 utilize "css/sgi" as the common source
directory. The "sgi" prefix originated from the System Guidance for
Infrastructure (SGI) and was initially associated with the SGI-575
platform. However, subsequent platforms released were under the Neoverse
Reference Design product name.

To align with the Neoverse Reference Design nomenclature, rename the
common source directory from "css/sgi" to "css/neoverse_rd" and update
all file prefixes from "sgi" to "nrd."

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: I3dcbb31b9ab202e82caf25218ba33c520dcea4e4
diff --git a/docs/about/maintainers.rst b/docs/about/maintainers.rst
index fed2a5c..3764084 100644
--- a/docs/about/maintainers.rst
+++ b/docs/about/maintainers.rst
@@ -524,7 +524,7 @@
 :|G|: `thomas-arm`_
 :|M|: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
 :|G|: `vijayenthiran-arm`_
-:|F|: plat/arm/css/sgi/
+:|F|: plat/arm/css/neoverse_rd/
 :|F|: plat/arm/board/rdn1edge/
 :|F|: plat/arm/board/rdn2/
 :|F|: plat/arm/board/rdv1/