ppc4xx: Move ppc4xx headers to powerpc include directory

This patch moves some ppc4xx related headers from the common include
directory (include/) to the powerpc specific one
(arch/powerpc/include/asm/). This way to common include directory is not
so cluttered with files.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/board/ml2/init.S b/board/ml2/init.S
index 9064d3b..91d053c 100644
--- a/board/ml2/init.S
+++ b/board/ml2/init.S
@@ -16,7 +16,7 @@
  *
  */
 
-#include <ppc4xx.h>
+#include <asm/ppc4xx.h>
 
 #include <ppc_asm.tmpl>
 #include <ppc_defs.h>