dt-bindings: fsl: convert fsl,layerscape-scfg to YAML

Convert the fsl,layerscape-scfg binding to the new YAML format.

In the device trees, the device node always have a "syscon"
compatible, which wasn't mentioned in the previous binding.

Also added, compared to the original binding, is the
interrupt-controller subnode as used in arch/arm/boot/dts/ls1021a.dtsi
as well as the litte-endian and big-endian properties.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220427075338.1156449-5-michael@walle.cc