use generic DMA mapping code in powerpc V4

[Christoph Hellwig]

On Sun, Feb 10, 2019 at 01:00:20PM +0100, Christian Zigotzky wrote:
> I tested the whole series today. The kernels boot and the P.A. Semi 
> Ethernet works! :-) Thanks a lot!
>
> I also tested it in a virtual e5500 QEMU machine today. Unfortunately the 
> kernel crashes.

This looks like a patch I fixed in mainline a while ago, but which
the powerpc tree didn't have yet.

I've cherry picked this commit
("swiotlb: clear io_tlb_start and io_tlb_end in swiotlb_exit")

and added it to the powerpc-dma.6 tree, please retry with that one.