mtd: spi-nor: drop unnecessary partition parser data

Now that the SPI-NOR/MTD framework pass the 'flash_node' through to the
partition parsing code, we don't have to do it ourselves.

Also convert 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: Boris Brezillon <boris.brezillon@free-electrons.com>