PCI: xilinx: Keep both legacy and MSI interrupt domain references

When built with MSI support, the legacy domain reference was being
overwritten with MSI.

Create two separate domains for MSI and legacy interrupts.

Signed-off-by: Bharat Kumar Gogada <bharatku@xilinx.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>