ARM: board: cm_fx6: fixup mtd partitions in the fdt

The cm-fx6 module has an on-board st,m25p compatible spi flash chip
used for U-Boot (binary & environment). Overwrite the partitions in
the device tree by the partition table provided in the mtdparts
environment variable, if it is set.

This allows to specify a kernel independent partitioning in the
environment and provides a convient way for the user to adapt the
partition table.

Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>