kbuild: Enable DT schema checks for %.dtb targets

It is possible to build a single dtb, but not with DT schema validation
enabled. Enable the schema validation to run for %.dtb and %.dtbo
targets. Anyone building a dtb for a specific platform *should* pay
attention to schema warnings.

This could be supported with a separate %.dt.yaml target instead.
However, the .dt.yaml format is considered an intermediate format and
could possibly go away at some point if schema checking is integrated
into dtc. Also, the plan is to enable the schema checks by default once
platforms are free of warnings, and this is a move in that direction.

Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20210913145146.766080-1-robh@kernel.org