It turns out that the 2GB contiguous DMA direct map window also needs

to be aligned on a 2GB boundary. Therefore the `add 512MB' bit used on
Octane does not give us a 0.5GB-2.5GB usable DMA range, but a 0.5GB-2GB
range; trying to use address in the 2GB-2.5GB range would cause PCI
DMA errors at the xbridge level.

There is no real benefit in using it, since this required us to keep
subtracting or adding 0.5GB when converting DMA address to physical
memory address or the other way around.

So stop using it; this makes a few parts of the code simpler (and until
bounce buffers are implemented, Octane systems will not use more than
1.5GB of memory).

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