dt-bindings: do not build processed-schema.yaml for 'make dt_binding_check'

Currently, processed-schema.yaml is always built, but it is actually
used only for 'make dtbs_check'.

'make dt_binding_check' uses processed-schema-example.yaml instead.

Build processed-schema.yaml only for 'make dtbs_check'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20200625170434.635114-3-masahiroy@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>