powerpc/ioda: Use ibm, supported-tce-sizes for IOMMU page size mask

At the moment we assume that IODA2 and newer PHBs can always do 4K/64K/16M
IOMMU pages, however this is not the case for POWER9 and now skiboot
advertises the supported sizes via the device so we use that instead
of hard coding the mask.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>