dtc: turn off dtc unit address warnings by default

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
(cherry picked from commit bc553986)
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

Change-Id: Ief2e68988f6cf32cb4e98f489fa4079f523c0887