[POWERPC] scc_sio: Fix link failure

scc_sio.o should only be built if the txx9 serial driver is actually
built into the kernel.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>

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Signed-off-by: Paul Mackerras <paulus@samba.org>