arm: fix location of some include files

The recent rearrangement of include files had some minor errors:
 devices.h is not ARM specific and should not be in arm/
 arm.h should be in arm/

Move these two headers to correct this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/hw/ppc/virtex_ml507.c b/hw/ppc/virtex_ml507.c
index 92b4394..6728ba7 100644
--- a/hw/ppc/virtex_ml507.c
+++ b/hw/ppc/virtex_ml507.c
@@ -27,7 +27,7 @@
 #include "hw/char/serial.h"
 #include "hw/block/flash.h"
 #include "sysemu/sysemu.h"
-#include "hw/arm/devices.h"
+#include "hw/devices.h"
 #include "hw/boards.h"
 #include "sysemu/device_tree.h"
 #include "hw/loader.h"