kbuild: clean up rule_dtc_dt_yaml

Commit 3a2429e1 ("kbuild: change if_changed_rule for multi-line
recipe") and commit 4f0e3a57 ("kbuild: Add support for DT binding
schema checks") came in via different sub-systems.

This is a follow-up cleanup.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>