common: Move testdram() into init.h

This function is called during init so move it to the init header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/freescale/m54418twr/m54418twr.c b/board/freescale/m54418twr/m54418twr.c
index a4943e7..8c90cfe 100644
--- a/board/freescale/m54418twr/m54418twr.c
+++ b/board/freescale/m54418twr/m54418twr.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <init.h>
 #include <spi.h>
 #include <asm/io.h>
 #include <asm/immap.h>