libata: pata_platform: Support polling-mode configuration.

Some SH boards (old R2D-1 boards) have generally not had working CF
under libata, due to both buswidth issues (handled by Aoi Shinkai
in 43f4b8c7), and buggy interrupt
controllers. For these sorts of boards simply disabling the IRQ and
polling ends up working fine.

This conditionalizes the IRQ resource for pata_platform and lets
platforms that want to use polling mode simply omit the resource
entirely.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>