dt-bindings: ehci/ohci: Allow iommus property

A OHCI/EHCI controller could be behind an IOMMU, in which case an iommus
property assigns the stream ID for this device.

Allow that property in the DT bindings to fix a complaint about the Arm Juno
board's DTS file.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>