kbuild: Enable dtc 'unit_address_format' warning by default

With all the 'unit_address_format' warnings fixed, enable the warning by
default.

Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: linux-kbuild@vger.kernel.org
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20210913192816.1225025-9-robh@kernel.org