PCI: cadence: Use AXI region 0 to signal interrupts from EP

PCI: cadence: Use AXI region 0 to signal interrupts from EP

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: 904bb5a93cbe ("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>