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 <marc.zyngier@arm.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Cc: Yijing Wang <wangyijing@huawei.com>
Cc: Ma Jun <majun258@huawei.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Duc Dang <dhdang@apm.com>
Cc: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: http://lkml.kernel.org/r/1438091186-10244-16-git-send-email-marc.zyngier@arm.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>