powerpc/vio: Remove dma not supported warnings

During boot we see a number of these warnings:

vio 30000000: Warning: IOMMU dma not supported: mask 0xffffffffffffffff, table unavailable

The reason for this is that we set IOMMU properties for all VIO
devices even if they are not DMA capable.

Only set DMA ops, table and mask for devices with a DMA window.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>