irqchip/gic-v3-its: Allow LPI invalidation via the DirectLPI interface

We currently don't make much use of the DirectLPI feature, and it would
be beneficial to do this more, if only because it becomes a mandatory
feature for GICv4.1.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20191027144234.8395-4-maz@kernel.org
Link: https://lore.kernel.org/r/20191108165805.3071-4-maz@kernel.org