mtd: drop unnecessary partition parser data

We should assign the MTD dev.of_node instead of the parser data field.
This gets us the equivalent partition parser behavior with fewer special
fields and parameter passing.

Also convert several of these to mtd_device_register(), since we don't
need the 2nd and 3rd parameters anymore.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>