mtd: ubi: wire-up parent MTD device

Wire up the device parent pointer of UBI devices to their lower MTD
device, typically an MTD partition or whole-chip device.

The most noticeable change is that in sysfs, previously ubi devices
would be could in /sys/devices/virtual/ubi while after this change they
would be correctly attached to their parent MTD device, e.g.

/sys/devices/platform/1100d000.spi/spi_master/spi1/spi1.0/mtd/mtd2/ubi0.

Locating UBI devices using /sys/class/ubi/ of course still works as
well.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Richard Weinberger <richard@nod.at>