libpayload/ahci: Fix a warning by decompartmentalise the AHCI driver.

Decompartmentalise AHCI driver into two parts, ATA and ATAPI. Add a few
superficial comments while here. This also fixes a compiler warning.

Change-Id: Ia1fd545b39868a81cbc311f6ffc786f9f1f61415
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/4783
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
6 files changed