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>