mtd: update documentation of mtd_device_parse_register()

In the commit 2c77c57d ("mtd: move code adding master MTD out of
mtd_add_device_partitions()") behavior of mtd_device_parse_register()
has very slightly changed. It's a pretty non-significant order change
to match updated function behavior.

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