[POWERPC] Split setup of IOMMU stab and ptab, allocate dynamic/fixed ptabs separately

Currently the cell IOMMU code allocates the entire IOMMU page table in a
contiguous chunk. This is nice and tidy, but for machines with larger
amounts of RAM the page table allocation can fail due to it simply being
too large.

So split the segment table and page table setup routine, and arrange to
have the dynamic and fixed page tables allocated separately.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>