powerpc/powernv/npu: Add PE to PHB's list

Before commit 3e68dc57 "powerpc/powernv: Remove DMA32 PE list", NPU PEs
were linked to the NPU PHB via phb->ioda.pe_dma_list; after that fix,
the phb->ioda.pe_list is used.

During the pe_dma_list removal, list_add_tail(&phb->ioda.pe_dma_list)
was removed, however no list_add() was added so does this patch.

Fixes: 3e68dc57219a ("powerpc/powernv: Remove DMA32 PE list")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>