booke206: fix booke206_tlbnps for mav 2.0

This fixes booke206_tlbnps for MAV 2.0 by checking the MMUCFG register and
return directly the right tlbnps instead of computing it from non existing
field.

Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
