In order to avoid leaking vgic_irq structures on teardown, we need to
drop all references to LPIs before deallocating the cache itself.

This is done by invalidating the cache on vgic teardown.
Tested-by: NAndre Przywara <andre.przywara@arm.com>
Reviewed-by: NEric Auger <eric.auger@redhat.com>
Signed-off-by: NMarc Zyngier <maz@kernel.org>