dt-bindings: mfd: Add bindings for sl28cpld

Add a device tree bindings for the board management controller found on
the Kontron SMARC-sAL28 board.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>