mtd: ofpart: add of_match_table with "fixed-partitions"

This allows using this parser with any flash driver that takes care of
setting of_node (using mtd_set_of_node helper) correctly. Up to now
support for "fixed-partitions" DT compatibility string was working only
with flash drivers that were specifying "ofpart" (manually or by letting
mtd use the default set of parsers).

This matches existing bindings documentation.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Brian Norris <computersforpeace@gmail.com>
Tested-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>