kbuild: determine the output format of DTC by the target suffix

cmd_dtc takes the additional parameter $(2) to select the target
format, dtb or yaml. This makes things complicated when it is used
with cmd_and_fixdep and if_changed_rule. I actually stumbled on this.
See commit 3d4b2238 ("kbuild: fix DT binding schema rule again to
avoid needless rebuilds").

Extract the suffix part of the target instead of passing the parameter.
Fortunately, this works for both $(obj)/%.dtb and $(obj)/%.dt.yaml .

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>