powerpc/dma: move pci_dma_dev_setup_swiotlb to fsl_pci.c

pci_dma_dev_setup_swiotlb is only used by the fsl_pci code, and closely
related to it, so fsl_pci.c seems like a better place for it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>