Probe for the 'msi-ranges' property, and implement the MSI support in the
form of the usual two-level hierarchy.

Note that contrary to the wired interrupts, MSIs are shared among all the
ports.

Link: https://lore.kernel.org/r/20210929163847.2807812-8-maz@kernel.orgTested-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>