powerpc/dart_iommu: Support for 64-bit iommu bypass window on PCIe

The PCI-Express bus off the U4/CPC945 bridge supports direct DMA to
all of memory, bypassing the DART iommu, for 64-bit capable devices.

This adds support for it on Bimini and Apple Quad G5's in order to
improve DMA performances of cards using that slot (the x16 graphics
slot). Tested with an Intel ixgbe 10GE card.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>