ACPI/IORT: Make platform devices initialization code SMMU agnostic

The way current IORT code initializes platform devices for SMMU nodes
is somewhat tied (mostly for naming convention) to the SMMU nodes
themselves but it need not be in that it is completely generic and
can easily be made so by structures renaming and code reshuffling.

Rework IORT platform devices initialization code to make the functions
and data structures SMMU agnostic.

No functional changes intended.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>