arm64: dts: xilinx: Align IOMMU nodename with dtschema

Fix dtschema validator warnings like:
    smmu@fd800000: $nodename:0: 'smmu@fd800000' does not match '^iommu@[0-9a-f]*'
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200629081744.13916-1-krzk@kernel.org

arch/arm64/boot/dts/xilinx/zynqmp.dtsi

@@ -611,7 +611,7 @@ sdhci1: mmc@ff170000 {
 			power-domains = <&zynqmp_firmware PD_SD_1>;
 		};
 
-		smmu: smmu@fd800000 {
+		smmu: iommu@fd800000 {
 			compatible = "arm,mmu-500";
 			reg = <0x0 0xfd800000 0x0 0x20000>;
 			status = "disabled";