mtd: get rid of the mtd_add_device_partitions()

This simplifies code a bit by:
1) Avoiding an extra (tiny) function
2) Checking for amount of parsed (found) partitions just once
3) Avoiding clearing/filling struct mtd_partitions manually

With this commit proper functions are called directly from the
mtd_device_parse_register(). It doesn't need to use minor tricks like
memsetting struct to 0 to trigger an expected
mtd_add_device_partitions() behavior.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>