mtd: ofpart: add ofoldpart alias

ofpart.ko also provides ofoldpart MTD parser. Add respective
MODULE_ALIAS("ofoldpart"); declaration.

Artem: improve the comment

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>