powerpc/eeh: Export eeh_iommu_group_to_pe()

The function is used by VFIO driver, which might be built as a
dynamic module. So it should be exported.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>