w1: Drop dm.h header file

This header file should not be included in other header files. Remove it
and use a forward declaration instead.

Signed-off-by: Simon Glass <sjg@chromium.org>

board/atmel/common/board.c

@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <dm.h>
 #include <env.h>
 #include <w1.h>
 #include <w1-eeprom.h>