dm: core: Drop dm.h header file from dm-demo.h

This header file should not be included in other header files. Remove it
and add it to the cmd file instead.

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

cmd/demo.c

@@ -8,6 +8,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <dm.h>
 #include <dm-demo.h>
 #include <mapmem.h>
 #include <asm/io.h>

include/dm-demo.h

@@ -6,8 +6,6 @@
 #ifndef __DM_DEMO_H
 #define __DM_DEMO_H
 
-#include <dm.h>
-
 /**
  * struct dm_demo_pdata - configuration data for demo instance
  *