board: sl28: move to OF_UPSTREAM

Use the new device devicetree files in dts/upstream/ and delete the old
ones. Still keep the -u-boot.dtsi with all u-boot specifics around.

There is one catch and that is fsl-ls1028a-kontron-sl28-var3.dts which
is not available upstream (yet!). For now, the base dts is used for this
variant as this only differ in the compatible and the (human readable)
model name.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Peng Fan <peng.fan@nxp.com>