ahci: add port I/O index-data pair

ahci: add port I/O index-data pair

Implement an I/O space index-data register pair as defined by the AHCI
spec, including the corresponding SATA PCI capability and BAR.

This allows real-mode code to access the AHCI registers; real-mode
code cannot address the memory-mapped register space because it is
beyond the first megabyte.

Signed-off-by: Daniel Verkamp <daniel@drv.nu>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>