rpi3: Move shared rpi3 files into common directory

To be able to share code more easily between the existing Raspberry Pi 3
and the upcoming Raspberry Pi 4 platform, move some code which is not
board specific into a "common" directory.

Change-Id: I9211ab2d754b040128fac13c2f0a30a5cc8c7f2c
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/rpi/rpi3/rpi3_bl31_setup.c b/plat/rpi/rpi3/rpi3_bl31_setup.c
index 2f1bc64..a9efc52 100644
--- a/plat/rpi/rpi3/rpi3_bl31_setup.c
+++ b/plat/rpi/rpi3/rpi3_bl31_setup.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -15,7 +15,7 @@
 #include <lib/xlat_tables/xlat_tables_defs.h>
 #include <plat/common/platform.h>
 
-#include "rpi3_private.h"
+#include <rpi_shared.h>
 
 /*
  * Placeholder variables for copying the arguments that have been passed to