genirq/msi: Provide struct msi_map

A simple struct to hold a MSI index / Linux interrupt number pair. It will
be returned from the dynamic vector allocation function and handed back to
the corresponding free() function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221124232326.326410494@linutronix.de