mtd: rename "ofpart" parser to "fixed-partitions" as it fits it better

Type "ofpart" means that OF should be used to get partitioning info and
this driver supports "fixed-partitions" binding only. Renaming it should
lead to less confusion especially when parsers for new compatibility
strings start to appear.

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