powerpc/powernv/npu: Add set/unset window helpers

The upcoming NVLink passthrough support will require NPU code to cope
with two DMA windows.

This adds a pnv_npu_set_window() helper which programs 32bit window to
the hardware. This also adds multilevel TCE support.

This adds a pnv_npu_unset_window() helper which removes the DMA window
from the hardware. This does not make difference now as the caller -
pnv_npu_dma_set_bypass() - enables bypass in the hardware but the next
patch will use it to manage TCE table lists for TCE Kill handling.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-By: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>