Do not compile nuvoton superio for all board

The nuvoton WPCM450 code is compiled for all boards regardless of
whether or not they use it. Compile it only for boards needing it.

Change-Id: Iaf4cf2c479eb3238863f0771be799f02a8cc3421
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/129
Tested-by: build bot (Jenkins)
Reviewed-by: Kerry She <shekairui@gmail.com>
Reviewed-by: Marc Jones <marcj303@gmail.com>