ahci: Get rid of host->iomap usage

Currently the driver uses host->iomap to store all the iomapped BARs
of a PCI device (while AHCI devices actually use just a single memory
window).

We're going to teach AHCI to work with non-PCI buses, so there are two
options to make this work:

1. "fake" host->iomap array for non-PCI devices, and place the needed
   address at iomap[AHCI_PCI_BAR];
2. Get rid of host->iomap usage, instead introduce a private mmio
   field.

This patch implements the second option.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>