[libata] pata_sil680: Add MMIO support

This patch adds MMIO support to the pata_sil680 for taskfile IOs,
based on what the old siimage does.

I haven't bothered changing the chip setup stuff from PCI config
cycles to MMIO though (siimage does it), I don't think it matters,
I've only adapted it to use MMIO for taskfile accesses.

I've tested it on a Cell blade and it seems to work fine.

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