libata:pata_atiixp: Don't use unconnected secondary port on SB600

libata:pata_atiixp: Don't use unconnected secondary port on SB600

    The AMD SB600 southbridge has an PATA IDE interface, but the
    secondary port has no physical connections, so is disabled in
    the PCI header which makes it appear as a legacy port.

    On most systems this causes no trouble, but the Amigaone X1000 has
    an SB600 connected to a PowerPC SoC PCI-e root port, with an
    emulated ISA bus. On this system a kernel panic occurs at boot
    time during device attach for the secondary port.

    Mark the port as 'dummy' to prevent this. As a bonus, disabling
    this will slightly speed up booting on PC systems using an
    SB600 as they will now skip 2 known empty ports.

    Signed-off-by: Darren Stevens <Darren@stevens-zone.net>
Signed-off-by: Tejun Heo <tj@kernel.org>