refactor(sgi): move neoverse_rd out of css

Currently, neoverse_rd is hosted under the "css" directory. However,
"css" directory is more relevant for hosting css definitions and
corresponding sources. Since neoverse_rd hosts source and header for css
and soc, move neoverse_rd from css to board folder. Consolidate common
sources and headers under neoverse_rd/common. Additionally, group RD-V1,
RD-V1-MC, RD-N2, RD-N1-Edgex2 and SGI-575 within neoverse_rd/platform.
With the changes in this commit, the tree view would look as follows:

plat/arm/board/neoverse_rd/
├── common
│   ├── arch
│   ├── include
│   └── ras
└── platform
    ├── rdn1edge
    ├── rdn2
    ├── rdv1
    ├── rdv1mc
    └── sgi575

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: Iaccc86bc9d415f5c045c834902241fcf3c00277b
diff --git a/plat/arm/board/neoverse_rd/platform/rdn2/rdn2_err.c b/plat/arm/board/neoverse_rd/platform/rdn2/rdn2_err.c
new file mode 100644
index 0000000..d712645
--- /dev/null
+++ b/plat/arm/board/neoverse_rd/platform/rdn2/rdn2_err.c
@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2020-2024, Arm Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <plat/arm/common/plat_arm.h>
+
+/*
+ * rdn2 error handler
+ */
+void __dead2 plat_arm_error_handler(int err)
+{
+	while (1) {
+		wfi();
+	}
+}