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