iommu/arm-smmu-qcom: Don't build debug features as a kernel module

The Qualcomm TBU debug support introduced by 414ecb03
("iommu/arm-smmu-qcom-debug: Add support for TBUs") provides its own
driver initialisation function, which breaks the link when the core SMMU
driver is built as a module:

  ld.lld: error: duplicate symbol: init_module
  >>> defined at arm-smmu.c
  >>>            drivers/iommu/arm/arm-smmu/arm-smmu.o:(init_module)
  >>> defined at arm-smmu-qcom-debug.c
  >>>            drivers/iommu/arm/arm-smmu/arm-smmu-qcom-debug.o:(.init.text+0x4)

Since we're late in the cycle, just make the debug features depend on a
non-modular SMMU driver for now while the initialisation is reworked to
hang off qcom_smmu_impl_init().

Signed-off-by: Will Deacon <will@kernel.org>