powerpc/fsl-pci: enable SWIOTLB in function setup_pci_atmu

This function contains all the stuff we need to check if SWIOTLB
should be enabled or not. So it is more convenient to enable
the SWIOTLB here than later.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>