arm64: dts: fsl-ls1028a-kontron-sl28: move MTD partitions

Move the MTD partitions to the partitions subnode. This is the new way
to specify the partitions, see
  Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>