mtd: make ofpart buildable as a separate module

As ofpart now uses a standard mtd partitions parser interface, make it
buildable as a separate module. Also provide MODULE_DESCRIPTION and
MODULE_AUTHOR for this module.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>