libata: add init helpers including ata_pci_prepare_native_host()

These will be used to convert LLDs to new init model.

* Add irq_handler field to port_info.  In new init model, requesting
  IRQ is LLD's responsibility and libata doesn't need to know about
  irq_handler.  Most LLDs can simply register their irq_handler but
  some need different irq_handler depending on specific chip.  The
  added port_info->irq_handler field can be used by LLDs to select
  the matching IRQ handler in such cases.

* Add ata_dummy_port_info.

* Implement ata_pci_prepare_native_host(), a helper to alloc ATA host,
  acquire all resources and init the host in one go.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>