First cut at large page support on 40x

[Paul Mackerras]

Dan Malek writes:

> If you would allow these to stay, you wouldn't have to change any other
> mapping functions, like iopa().  It's only a couple of pages.......

Dan, you continue to surprise me.  You complain about a kB or two for
a device tree but happily throw away anything from 16kB to 128kB or
more just to simplify iopa, which should not be needed anyway.

Paul.

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