mtd: physmap_of: move parse_obsolete_partitions to become separate parser

Move parse_obsolete_partitions() to ofpart.c and register it as an
ofoldpart partitions parser.

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