Docs: dt: add PCI IOMMU map bindings

The existing IOMMU bindings are able to specify the relationship between
masters and IOMMUs, but they are insufficient for describing the general
case of hotpluggable busses such as PCI where the set of masters is not
known until runtime, and the relationship between masters and IOMMUs is
a property of the integration of the system.

This patch adds a generic binding for mapping PCI devices to IOMMUs,
using a new iommu-map property (specific to PCI*) which may be used to
map devices (identified by their Requester ID) to sideband data for the
IOMMU which they master through.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>