ARM: configs: cm_fx6: add mtd support

ARM: configs: cm_fx6: add mtd support

The cm-fx6 module has an on-board spi flash chip. Enable mtd support
and the mtdparts command. Also define a default partitioning, add
it to the default environment, and enable support to overwrite the
partitioning defined in a device tree by it. Finally, probe for the
chip on preboot to register the flash chip and, thus, establish the
connection between the mtd environment settings and the actual device.

These changes move the effective default partitioning from the device
tree shipped with the vendor kernels to U-Boot which becomes the single
point of definition for the partitioning for all device tree based
kernels (in particular, for the upstream Linux kernel which does not
have a default partitioning defined in its device tree).

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