Support for multiple AHCI controllers

[IanSeyler]

• [ ] AHCI driver must search for controllers via all possible PCI IDs and save BAR5 (AHCI Base Address Register) for each detected instead of just stopping at the first.
• [ ] Store information on if drives are detected on each controller. AHCI supports 32 devices via a controller as per the AHCI specs. Drive numbers will be assigned based on controller and port numbers. First port on first controller is drive 0. Last port on first controller is drive 31. First port on second controller is drive 32. And so on.
• [ ] Add call for drive info to return 512-byte identity data.
• [ ] Enough memory will need to be reserved for the structures needed.