linux-imx

genirq/msi: Make the .prepare callback reusable

The .prepare callbacks are so far only called from msi_domain_alloc_irqs.
In order to reuse that code, split that code and create a
msi_domain_prepare_irqs function that the existing code can call into.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>