The IRQ physical address is allocated from region 0, rather than
the highest region. Update the driver to reserve this region in
the bitmap and to use region 0 for all types of interrupt.

This corrects a problem which prevents the interrupt being
signalled correctly if using the first address in the AXI region,
since an offset of zero will always be mapped to region 0.

Fixes: 37dddf14 ("PCI: cadence: Add EndPoint Controller driver for Cadence PCIe controller")
Signed-off-by: Alan Douglas <adouglas@cadence.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>