refactor(sgi): regroup "sgi" and "rdinfra" to "neoverse_rd"

Currently, reference design platforms such as RD-N2, RD-N1-Edge, RD-V1,
and SGI-575 utilize "sgi/common" as the common source directory. The
"sgi" prefix originated from the System Guidance 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, regroup all
common and board files within neoverse_rd directory. Consolidate common
sources and headers under neoverse_rd/common. Board files for RD-V1,
RD-N2, RD-N1-Edge and SGI-575 are moved to neoverse_rd/platform. With
the changes in this commit, the tree view would look as follows:

├── neoverse_rd
│   ├── common
│   │   ├── arch
│   │   └── include
│   └── platform
│       ├── rdn1edge
│       ├── rdn2
│       ├── rdv1
│       └── sgi575

Additionally, update all file prefixes from "sgi" to "nrd."

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: I07e2af143fad82e48172612fe0e28db6464fa901
34 files changed