So far, the iommu code was hardwired to a linear mapping
between 0x20000000 and 0x40000000, so it could only support
512MB of RAM.

This patch still keeps the linear mapping, but looks for
proper ibm,dma-window properties to set up larger windows,
this makes the maximum supported RAM size 2GB.

If there is anything unusual about the dma-window properties,
we fall back to the old behavior.

We also support switching off the iommu completely now
with the regular iommu=off command line option.
Signed-off-by: Arnd Bergmann <arndb@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>