Add the basic data structure that expresses an MSI to LPI
translation as well as the allocation/release hooks.

The size of the cache is arbitrarily defined as 16*nr_vcpus.
Tested-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>