The newly added dtc warning to check DT unit-address without reg
property and vice-versa generates lots of warnings. Turn off the check
unless building with W=1 or W=2.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Michal Marek <mmarek@suse.com>
Cc: linux-kbuild@vger.kernel.org