PCI/MSI: Allow msi_domain lookup using the host bridge node

PCI/MSI: Allow msi_domain lookup using the host bridge node

A number of platforms do not need to use the msi-parent property,
as the host bridge itself provides the MSI controller.

Allow this configuration by performing an irq domain lookup based
on the host bridge node if it doesn't have a valid msi-parent property.

Acked-by: Bjorn Helgaas <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Cc: <[email protected]>
Cc: Yijing Wang <[email protected]>
Cc: Ma Jun <[email protected]>
Cc: Lorenzo Pieralisi <[email protected]>
Cc: Duc Dang <[email protected]>
Cc: Hanjun Guo <[email protected]>
Cc: Jiang Liu <[email protected]>
Cc: Jason Cooper <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>