powerpc/iommu: Use permission-specific DEVICE_ATTR variants

Use DEVICE_ATTR_RW for read-write attributes.  This simplifies the
source code, improves readbility, and reduces the chance of
inconsistencies.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>