bus: fsl-mc-msi: Simplify MSI descriptor handling

Let the MSI irq domain code handle descriptor allocation and free.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211206210748.847219474@linutronix.de