mtd: mtdpart: default to cmdlinepart, NULL partitions probing

Lots of MTD devices default to cmdlinepart, NULL as partition parsing
order. Make it a default.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>