arm: gic-v3-its: Use shared pages wherever needed

GICv3-ITS is emulated by the host and thus we should allocate shared pages for
access by the host. Make sure the allocations are shared.

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Joey Gouly <joey.gouly@arm.com>