powerpc: Add 86xx support for SWIOTLB

This is the final bit of code to allow enabling swiotlb on
mpc86xx.  The platform-specific code is very small and consists
of enabling SWIOTLB in the config file, registering the
swiotlb_setup_bus_notifier initcall, and setting pci_dma_ops
to point to swiotlb_pci_dma_ops if we have more memory than
can be mapped by the inbound PCI windows.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>