ipq40xx: utilize nvmem-cells for openmesh,a42

ipq40xx: utilize nvmem-cells for openmesh,a42

The calibration data and mac addresses on this device are stored in the
0:ART partition. It is therefore possible to move the code to handle them
directly to the devicetree instead of the various scripts.

But the actual relevant information about the partition layout is provided
by the bootloader via bootargs (mtdparts) and not via the devicetree
itself. Instead of using a fixed-partition template, the mtd dynamic
partitions support from the upstream kernel is used.

Reported-by: Robert Marko <robert.marko@sartura.hr>
Reviewed-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>