irqchip/gic-v3-its: Parse new version of msi-parent property

Now that 126b16e2 ("Docs: dt: add generic MSI bindings")
has made it into the tree, the time has come to get rid of the
old hack, and to parse msi-parent in its full glory.

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