Kbuild: enable -Wunused-macros warning for "make W=2"

We have lots of dead defines and macros in drivers, lets offer users a way
to detect and eventually remove them.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>