While msi-parent is used to point to the MSI controller that
works for all the devices behind a root complex, it doesn't
allow configurations where each individual device can be routed
to a separate MSI controller.

The msi-map property provides this flexibility (and much more),
so let's add a utility function that parses it, and return the
corresponding MSI domain.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>