dtc gained new warnings checking PCI and simple buses, unit address
formatting, and stricter node and property name checking. Disable the
new dtc warnings by default as there are 1000s. As before, warnings are
enabled with W=1 or W=2. The strict node and property name checks are a
bit subjective, so they are only enabled for W=2.
Signed-off-by: Rob Herring <robh@kernel.org>