[POWERPC] Add support to cell_iommu_setup_page_tables() for multiple windows

Add support to cell_iommu_setup_page_tables() for handling two windows,
the dynamic window and the fixed window.  A fixed window size of 0
indicates that there is no fixed window at all.

Currently there are no callers who pass a non-zero fixed window, but the
upcoming fixed IOMMU mapping patch will change that.

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