Documentation: fsl-mc: add iommu-map device-tree binding for fsl-mc bus

The existing IOMMU bindings cannot be used to specify the relationship
between fsl-mc devices and IOMMUs. This patch adds a generic binding for
mapping fsl-mc devices to IOMMUs, using iommu-map property.

Signed-off-by: Nipun Gupta <nipun.gupta@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>