pata_sil680: Fix build on arch/ppc

Commit 0f436eff

 breaks build on
arch/ppc as it doesn't implement the machine_is() macro.

This fixes it by using CONFIG_PPC_MERGE instead which represents
arch/powerpc only, while CONFIG_PPC is set for both.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>