wdt: Drop dm.h header file

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

Also remove the other headers that are not needed, since the inline code
was removed in a recent commit:

   b4d9452c4 ("watchdog: move initr_watchdog() to wdt-uclass.c")

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

include/wdt.h

@@ -6,9 +6,7 @@
 #ifndef _WDT_H_
 #define _WDT_H_
 
-#include <dm.h>
-#include <log.h>
-#include <dm/read.h>
+struct udevice;
 
 /*
  * Implement a simple watchdog uclass. Watchdog is basically a timer that