dm: Fix util.h's broken include guard

Fix up the header's include guard to contain the definition of
dm_priv_to_rw(), which was erroneously added outside of it, by moving
its #endif to the end of the file (i.e. where it belongs). This removes
the risk of compilation errors resulting from the redefinition of that
function where the header might have been (indirectly) included more
than once.

Fixes: cfb9c9b7 ("dm: core: Use separate priv/plat data region")
Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
Cc: Simon Glass <sjg@chromium.org>