board: tqma6: change U-Boot device tree fragments

board: tqma6: change U-Boot device tree fragments

We will need GPIO to distinguish between SOM variants with / without
workaround for ENET errata. This is necessary to implement the
appropriate solution and remove code intended for the non-DM use case.
To support reuse, split them in SOM and baseboard specific parts.

Prepare fragments for use with SPL.

Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
Signed-off-by: Max Merchel <Max.Merchel@ew.tq-group.com>