kbuild: Build DT binding examples with dtc warnings enabled

Now that we have a separate rule for DT binding examples, we can customize
the dtc options. Let's adjust the dtc warnings to me more strict by
default so the examples get cleaned up as they get converted to schema.

Leaving 'avoid_unnecessary_addr_size' and 'graph_child_address' warnings
disabled as examples tend to be incomplete and they generates a lot of
warnings.

Co-developed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>