arm64: dts: ti: k3-j721e-main: rename smmu node to iommu

Rename smmu node to iommu to fix dtbs_check warning:
 k3-j721e-common-proc-board.dt.yaml: smmu@36600000: $nodename:0: 'smmu@36600000' does not match '^iommu@[0-9a-f]*'

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>