ACPI/IORT: Fix build error when IOMMU_SUPPORT is disabled

If IOMMU_SUPPORT is not enabled (and therefore IOMMU_API is not
selected), struct iommu_fwspec is an empty struct and
IOMMU_FWSPEC_PCI_RC_ATS is not defined, resulting in the following
compilation errors:

drivers/acpi/arm64/iort.c: In function iort_iommu_configure:
drivers/acpi/arm64/iort.c:1079:21: error: struct iommu_fwspec has no member named flag:
    dev->iommu_fwspec->flags |= IOMMU_FWSPEC_PCI_RC_ATS;
                     ^~
drivers/acpi/arm64/iort.c:1079:32: error: IOMMU_FWSPEC_PCI_RC_ATS
undeclared (first use in this function)
    dev->iommu_fwspec->flags |= IOMMU_FWSPEC_PCI_RC_ATS;
                                ^~~~~~~~~~~~~~~~~~~~~~~
drivers/acpi/arm64/iort.c:1079:32: note: each undeclared identifier is reported only once for each function it appears in

Move iort_iommu_configure() (and the helpers functions it relies on)
into CONFIG_IOMMU_API preprocessor guarded code so that when
CONFIG_IOMMU_SUPPORT is not enabled we prevent compili...