[POWERPC] Cell IOMMU fixed mapping support

This patch adds support for setting up a fixed IOMMU mapping on certain
cell machines.  For 64-bit devices this avoids the performance overhead of
mapping and unmapping pages at runtime.  32-bit devices are unable to use
the fixed mapping.

The fixed mapping is established at boot, and maps all of physical memory
1:1 into device space at some offset.  On machines with < 30 GB of memory
we setup the fixed mapping immediately above the normal IOMMU window.

For example a machine with 4GB of memory would end up with the normal
IOMMU window from 0-2GB and the fixed mapping window from 2GB to 6GB. In
this case a 64-bit device wishing to DMA to 1GB would be told to DMA to
3GB, plus any offset required by firmware.  The firmware offset is encoded
in the "dma-ranges" property.

On machines with 30GB or more of memory, we are unable to place the fixed
mapping above the normal IOMMU window as we would run out of address space.
Instead we move the normal IOMMU window to coincide with the hash page
table, this region does not need to be part of the fixed mapping as no
device should ever be DMA'ing to it.  We then setup the fixed mapping
from 0 to 32GB.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>