irqchip: gicv3-its: Use non-cacheable accesses when no shareability

If the ITS or the redistributors report their shareability as zero,
then it is important to make sure they will no generate any cacheable
traffic, as this is unlikely to produce the expected result.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1427465705-17126-5-git-send-email-marc.zyngier@arm.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>