irqchip/gicv3-its: Disable the ITS before initializing it

When starting a kexec/kdump kernel, the GIC ITS will already have been
enabled.  According to the ARM Generic Interrupt Controller
Architecture Specification (GIC architecture Version 3.0 and version
4.0), writing to GITS_BASER<n> or GITS_CBASER is "UNPREDICTABLE" when
the ITS is enabled.  On Cavium Thunder systems, this prevents the ITS
from being initializing in the kexec/kdump kernel, resulting in
failure to register/enable interrupts for all devices.

The fix is to disable the ITS if it is not already in the disabled
state.  This allows the ITS to be properly initialized and then
re-enabled in the kexec/kdump kernel.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>