ARM: dts: imx6ul-kontron: Simplify devicetree structure

There is no need in differentiating between the different SoM versions
that only differ in memory size, etc. It's much cleaner to simplify
the devicetrees and let them be more generic.

If necessary the bootloader will update the devicetree dynamically
to contain the correct size of the memory.

We also get rid of the N6xxx notation in the compatibles and file
names, as they are not really used anymore and often result in
confusion.

This is a breaking change, but the impact shouldn't be too big and it
makes usage and maintenance easier in the future.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>