powerpc/cell: Use for_each_of_range() iterator

Simplify the cell_iommu_get_fixed_address() dma-ranges parsing to use
the for_each_of_range() iterator.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241106212647.341857-1-robh@kernel.org