genirq/msi: Add helper for creating MSI-parent irq domains

Creating an irq domain that serves as an MSI parent requires
a substantial amount of esoteric boiler-plate code, some of
which is often provided twice (such as the bus token).

To make things a bit simpler for the unsuspecting MSI tinkerer,
provide a helper that does it for them, and serves as documentation
of what needs to be provided.

Signed-off-by: Marc Zyngier <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

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