platform-msi: Let core code handle MSI descriptors

Use the core functionality for platform MSI interrupt domains. The platform
device MSI interrupt domains will be converted in a later step.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211206210748.903173257@linutronix.de