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>
diff --git a/cmd/w1.c b/cmd/w1.c
index 92be1f2..459094b 100644
--- a/cmd/w1.c
+++ b/cmd/w1.c
@@ -6,6 +6,7 @@
  */
 #include <common.h>
 #include <command.h>
+#include <dm.h>
 #include <w1.h>
 #include <w1-eeprom.h>
 #include <dm/device-internal.h>