irqchip/gic-v3-its: Add a cache invalidation right after vPE unmapping

In order to be able to manipulate the VPT once a vPE has been
unmapped, perform the required CMO to invalidate the CPU view
of the VPT.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Shenming Lu <lushenming@huawei.com>
Link: https://lore.kernel.org/r/20210322060158.1584-2-lushenming@huawei.com