kbuild: fix DT binding schema rule to detect command line changes

kbuild: fix DT binding schema rule to detect command line changes

This if_change_rule is not working properly; it cannot detect any
command line change.

The reason is because cmd-check in scripts/Kbuild.include compares
$(cmd_$@) and $(cmd_$1), but cmd_dtc_dt_yaml does not exist here.

For if_change_rule to work properly, the stem part of cmd_* and rule_*
must match. Because this cmd_and_fixdep invokes cmd_dtc, this rule must
be named rule_dtc.

Fixes: 780b40716c09 ("kbuild: Add support for DT binding schema checks")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>