As commit d0e628cd ("kbuild: doc: clarify the difference between
extra-y and always-y") explained, extra-y should be used for listing
the prerequisites of vmlinux.

These targets are not related to vmlinux. always-y is a better fix.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: NRob Herring <robh@kernel.org>