PCI/MSI: Use of_msi_get_domain instead of open-coded "msi-parent" parsing

Now that we have a function that implements the complexity of the
"msi-parent" property parsing, switch to that.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>