kbuild: remove meaningless parameter to $(call if_changed_rule,dtc)

This is a remnant of commit 78046fab ("kbuild: determine the output
format of DTC by the target suffix").

The parameter "yaml" is meaningless because cmd_dtc no loner takes $(2).

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>