libata: convert legacy PCI host handling to new init model

Convert legacy PCI host handling to alloc-init-register model.
ata_init_legacy_host(), ata_request_legacy_irqs() and
ata_pci_init_bmdma() are separated out and follow the new init model.

The two legacy handling functions use separate ata_legacy_devres
instead of generic devm_* resources.  This reduces devres overhead for
legacy hosts which was a bit high because it didn't use PCI/iomap
merged resoruces.

ata_pci_init_one() is rewritten in terms of the aboved functions but
native mode handling is still using the old method.  Conversion will
be completed when native mode handling is updated.

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