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:

   b4d9452c ("watchdog: move initr_watchdog() to wdt-uclass.c")

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