of/irq: Add support code for multi-parent version of "msi-parent"

of/irq: Add support code for multi-parent version of "msi-parent"

Since 126b16e2ad98 ("Docs: dt: add generic MSI bindings"),
the definition of "msi-parent" has evolved, while maintaining
some degree of compatibility. It can now express multiple MSI
controllers as parents, as well as some sideband data being
communicated to the controller.

This patch adds the parsing of the property, iterating over
the multiple parents until a suitable irqdomain is found.
It can also fallback to the original parsing if the old
binding is detected.

This support code gets used in the subsequent patches.

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