dt-bindings: Re-enable core schemas for dtbs_check

In commit 2ba06cd8 ("kbuild: Always validate DT binding examples"),
the core schemas (from dtschema repo) were inadvertently disabled for
dtbs_checks. Re-enable them.

Fixes: 2ba06cd8

 ("kbuild: Always validate DT binding examples")
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>