mtd: physmap-core: Prioritize ofparts for OF probe

Place the ofparts before RedBoot partitions in the OF probe
path: this makes it possible to override any existing
RedBoot partitions with fixed-partitions which may be necessary,
such as when you need to repartition the flash from Linux'
point of view but not necessary from the bootloaders point
of view.

This happens when a device such as Raidsonic IB-4220-B has
three partitions named "Kern", "Ramdisk" and "Application"
that we want to merge into one for more efficient use
of the flash memory in OpenWrt.

Signed-off-by: Linus Walleij <linusw@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>