x86, irq: Add data structure to keep AMD specific irq remapping information

Add a data structure to store information the IOMMU driver
can use to get from a 'struct irq_cfg' to the remapping
entry.

Signed-off-by: Joerg Roedel <joro@8bytes.org>
Acked-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>