dt-bindings: arm: fsl: add Kontron sl28 variant 1

Add the a new variant for the Kontron SMARC-sAL28 board.

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