IORT revision C introduced a mapping entry binding to describe ITS
device ID mapping for SMMUv3 MSI interrupts.

Enable the single mapping flag (ie that is used by SMMUv3 component for
its special index mappings) for the SMMUv3 node in the IORT mapping API
and add IORT code to handle special index mapping entry for the SMMUv3
IORT nodes to enable their MSI interrupts. In case the ACPICA for
SMMUv3 device ID mapping is not ready, use the ACPICA version as a guard
for function iort_get_id_mapping_index().
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
[lorenzo.pieralisi@arm.com: patch split, typos fixing, rewrote the log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>