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/netstal/common/fixed_sdram.c b/board/netstal/common/fixed_sdram.c
index 2f21fbb..51b34b2 100644
--- a/board/netstal/common/fixed_sdram.c
+++ b/board/netstal/common/fixed_sdram.c
@@ -19,7 +19,7 @@
  */
 
 #include <common.h>
-#include  <ppc4xx.h>
+#include  <asm/ppc4xx.h>
 #include  <asm/processor.h>
 #include  "nm.h"