irqchip/gicv3-its: Add platform MSI support

irqchip/gicv3-its: Add platform MSI support

In order to support non-PCI MSI with the GICv3 ITS, add the minimal
required entry points for the MSI domain (an msi_prepare implementation).

The rest is only boilerplate code to find the raw ITS domain.

Signed-off-by: Marc Zyngier <[email protected]>
Cc: <[email protected]>
Cc: Yijing Wang <[email protected]>
Cc: Ma Jun <[email protected]>
Cc: Lorenzo Pieralisi <[email protected]>
Cc: Duc Dang <[email protected]>
Cc: Hanjun Guo <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Jiang Liu <[email protected]>
Cc: Jason Cooper <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>