[POWERPC] Make cell IOMMU fixed mapping printk more useful

Currently the cell IOMMU fixed mapping just printks that it's been setup,
which is not particularly useful.  Much more interesting is the address
ranges for the different windows.  This adds one line to dmesg on a blade.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>