powerpc/powernv/ioda2: Add 256M IOMMU page size to the default POWER8 case

The sketchy bypass uses 256M pages so add this page size as well.

This should cause no behavioral change but will be used later.

Fixes: 477afd6ea6 "powerpc/ioda: Use ibm,supported-tce-sizes for IOMMU page size mask"
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>