[POWERPC] Split cell_iommu_setup_hardware() into two parts

Split cell_iommu_setup_hardware() into two parts.  Split the page table
setup into cell_iommu_setup_page_tables() and the bits that kick the
hardware into cell_iommu_enable_hardware().

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