irqchip/gicv3-its: Split PCI/MSI code from the core ITS driver

It is becoming obvious that having the PCI/MSI code in the same
file as the the core ITS code is giving people implementing non-PCI
MSI support the wrong kind of idea.

In order to make things a bit clearer, let's move the PCI/MSI code
out to its own file. Hopefully it will make it clear that whoever
thinks of hooking into the core ITS better have a very strong point.

We use a temporary entry point that will get removed in a subsequent
patch, once the proper infrastructure is added.

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/1438091186-10244-12-git-send-email-marc.zyngier@arm.com


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