iommu: Validate device tree node in dev_iommu_enable

iommu: Validate device tree node in dev_iommu_enable

Similar to pinctrl_select_state(), add dev_has_ofnode() check before doing the
real work. Device(scmi_base.0) does not have a real device node, ofnode_null()
is assigned as the device tree node for scmi base protocol device:
'commit 7eb4eb541c14 ("firmware: scmi: install base protocol to SCMI agent")'

However with recent update in
'commit 0535e46d55d7 ("scripts/dtc: Update to upstream version v1.7.2-35-g52f07dcca47c")',
SPL panic in fdt_check_node_offset_()->fdt_next_tag(), because offset is -1
and SPL_OF_LIBFDT_ASSUME_MASK is 0xFF.

So need to validate device tree node.

Reported-by: Ye Li <ye.li@nxp.com>
Closes: https://lore.kernel.org/u-boot/939a9696-27fa-45a1-b428-feffe21ac6d5@oss.nxp.com/
Signed-off-by: Peng Fan <peng.fan@nxp.com>