common: Drop bootstage.h from common header

Move this fairly uncommon header out of the common header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/mips/lib/bootm.c b/arch/mips/lib/bootm.c
index f1db6d2..6a6764a 100644
--- a/arch/mips/lib/bootm.c
+++ b/arch/mips/lib/bootm.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <bootstage.h>
 #include <env.h>
 #include <image.h>
 #include <fdt_support.h>