powerpc/pseries/iommu: Define spapr_tce_table_group_ops only with CONFIG_IOMMU_API

powerpc/pseries/iommu: Define spapr_tce_table_group_ops only with CONFIG_IOMMU_API

The patch fixes the below warning:
  arch/powerpc/platforms/pseries/iommu.c:1824:37: warning:
  'spapr_tce_table_group_ops' defined but not used

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407020357.Hz8kQkKf-lkp@intel.com/
Fixes: b09c031d9433 ("powerpc/iommu: Move pSeries specific functions to pseries/iommu.c")
Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/172008854222.784.13666247605789409729.stgit@linux.ibm.com