plat/arm/sgi: move bl31_platform_setup to board file

For SGI-575 and RD platforms, move bl31_platform_setup handler to
individual board files to allow the platforms to perform board specific
bl31 setup.

Change-Id: Ia44bccc0a7f40a155b33909bcb438a0909b20d42
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
diff --git a/plat/arm/css/sgi/include/sgi_plat.h b/plat/arm/css/sgi/include/sgi_plat.h
new file mode 100644
index 0000000..a5fbded
--- /dev/null
+++ b/plat/arm/css/sgi/include/sgi_plat.h
@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2020, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef SGI_PLAT_H
+#define SGI_PLAT_H
+
+/* BL31 platform setup common to all SGI based platforms */
+void sgi_bl31_common_platform_setup(void);
+
+#endif /* SGI_PLAT_H */