powerpc/powernv/npu: Remove NPU DMA ops

The NPU IOMMU is setup to mirror the parent PCIe device IOMMU
setup. Therefore it does not make sense to call dma operations such as
dma_map_page(), etc. directly on these devices. The existing dma_ops
simply print a warning if they are ever called, however this is
unnecessary and the warnings are likely to go unnoticed.

It is instead simpler to remove these operations and let the generic
DMA code print warnings (eg. via a NULL pointer deref) in cases of
buggy drivers attempting dma operations on NVLink devices.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>