kbuild: Enable DT undocumented compatible checks

kbuild: Enable DT undocumented compatible checks

dt-validate has an option to warn on any compatible strings which don't
match any schema. The option has recently been improved to fix false
positives, so let's enable the option. This is useful for tracking
compatibles which are undocumented or not yet converted to DT schema.
Previously, the only check of undocumented compatible strings has been
an imperfect checkpatch.pl check.

The option is enabled by default for 'dtbs_check'. This will add more
warnings, but some platforms are down to only a handful of these
warnings (good job!).

There's about 100 cases in the binding examples, so the option is
disabled until these are fixed. In the meantime, they can be checked
with:

make DT_CHECKER_FLAGS=-m dt_binding_check

Cc: Maxime Ripard <mripard@kernel.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210311233640.1581526-2-robh@kernel.org