dt-bindings: mmc: renesas,sdhi: Add iommus property

The SDHI blocks on Renesas R-Car and RZ/G2 SoCs make use of IOMMU.

This patch fixes the below dtbs_check warnings:
arch/arm64/boot/dts/renesas/r8a774e1-hihope-rzg2h-ex-idk-1110wr.dtb: mmc@ee100000: Unevaluated properties are not allowed ('iommus' was unexpected)
	From schema: Documentation/devicetree/bindings/mmc/renesas,sdhi.yaml
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220831214314.7794-1-prabhakar.mahadev-lad.rj@bp.renesas.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/renesas,sdhi.yaml

@@ -89,6 +89,9 @@ properties:
         - tx
         - rx
 
+  iommus:
+    maxItems: 1
+
   power-domains:
     maxItems: 1