irqchip: gicv3-its: Define macros for GITS_CTLR fields

Define macros for GITS_CTLR fields to avoid using magic numbers.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Yun Wu <wuyun.wu@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1425659870-11832-11-git-send-email-marc.zyngier@arm.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>