[POWERPC] Fix Maple secondary IDE interrupt

The IDE driver will pick up the PCI IRQ for both channels on Maple
despite the fact that it's in legacy mode. This works around it by
"hiding" the PCI IRQ of the AMD8111 IDE controller when it's configured
in legacy mode on the Maple platform, thus causing the driver to call
pci_get_legacy_ide_irq() which will return the correct interrupts for
both channels.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>