common: powerpc: Move arch-specific headers

Set up a new asm/ppc.h header file to hold this arch-specific stuff. It
should not be in common.h. It probably should be refactored to use
asm/arch instead, but that is a job for the maintainer.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/powerpc/cpu/ppc4xx/reginfo.c b/arch/powerpc/cpu/ppc4xx/reginfo.c
index a42327e..59de04a 100644
--- a/arch/powerpc/cpu/ppc4xx/reginfo.c
+++ b/arch/powerpc/cpu/ppc4xx/reginfo.c
@@ -14,6 +14,7 @@
 #include <command.h>
 #include <asm/processor.h>
 #include <asm/io.h>
+#include <asm/ppc4xx.h>
 #include <asm/ppc4xx-uic.h>
 #include <asm/ppc4xx-emac.h>