dt-bindings: net: renesas,ether: Add iommus property

make dtbs_check:

arch/arm64/boot/dts/renesas/r8a77980-condor.dtb: ethernet@e7400000: 'iommus' does not match any of the regexes: '@[0-9a-f]$', 'pinctrl-[0-9]+'
from schema $id: http://devicetree.org/schemas/net/renesas,ether.yaml#

Ethernet Controllers on R-Car Gen2/Gen3 SoCs can make use of the IOMMU,
so add the missing iommus property.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@gmail.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/2ca890323477a21c22e13f6a1328288f4ee816f9.1729868894.git.geert+renesas@glider.be
Signed-off-by: Jakub Kicinski <kuba@kernel.org>