dt-bindings: arm: fsl: add Kontron sl28 boards

Add the Kontron SMARC-sAL28 board, its variants and combination with
carriers.

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