iommu: arm-smmu: fix compile error if CONFIG_PCI disabled

of_get_device_group() uses pci_bus_type, compilation error will
happen if CONFIG_PCI is disabled, add checks for this config.

Fixes: 6d696b37 ("iommu: arm-smmu: Support default domain attributes")
Change-Id: I7c5073b5bc5004600a4633b75fe7d5ea764cc656
Signed-off-by: Qingqing Zhou <qqzhou@codeaurora.org>