of/iommu: Make of_map_rid() PCI agnostic

of/iommu: Make of_map_rid() PCI agnostic

There is nothing PCI specific (other than the RID - requester ID)
in the of_map_rid() implementation, so the same function can be
reused for input/output IDs mapping for other busses just as well.

Rename the RID instances/names to a generic "id" tag.

No functionality change intended.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Joerg Roedel <jroedel@suse.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200619082013.13661-7-lorenzo.pieralisi@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>