powerpc/iommu: Fix initialisation of DART iommu table

Commit d0847757 switched the generic
powerpc iommu backend code to use the it_page_shift field to determine
page size. Commit 3a553170 should have
initiliased this field for all platforms, however the DART iommu table
code was not updated.

This commit initialises the it_page_shift field to 4K for the DART
iommu.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>