irqchip: GICv3: ITS command queue

The ITS is configured through a number commands that the driver
issues to the HW using a memory-based circular buffer.

This patch implements the subset of commands that are required
for Linux.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1416839720-18400-5-git-send-email-marc.zyngier@arm.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>

diff --cc drivers/irqchip/irq-gic-v3-its.c
Simple merge

diff --cc include/linux/irqchip/arm-gic-v3.h
Simple merge