dt-bindings: kbuild: Fix dt_binding_check on unconfigured build

dt-bindings: kbuild: Fix dt_binding_check on unconfigured build

The 'dt_binding_check' target shouldn't depend on the kernel
configuration, but it has since commit 604a57ba9781 ("dt-bindings:
kbuild: Add separate target/dependency for processed-schema.json").
That is because CHECK_DT_BINDING make variable was dropped, but
scripts/dtc/Makefile was missed. The CHECK_DTBS variable can be used
instead.

Reported-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Fixes: 604a57ba9781 ("dt-bindings: kbuild: Add separate target/dependency for processed-schema.json")
Signed-off-by: "Rob Herring (Arm)" <robh@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>