genirq/msi: Provide msi_domain_ops:: Post_free()

To prepare for removing the exposure of __msi_domain_free_irqs() provide a
post_free() callback in the MSI domain ops which can be used to solve
the problem of the only user of __msi_domain_free_irqs() in arch/powerpc.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20221111122014.063153448@linutronix.de