mtd: hide parse_mtd_partitions

There is no need to export parse_mtd_partitions() now , as it's fully handled
by registration functions. So move the definition to private header and
remove respective EXPORT_SYMBOL_GPL.

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