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

Convert the fsl,layerscape-dcfg 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.

One thing added here, compared to the original binding is the clock
controller subnode of the LS1028A SoC and its "simple-mfd" compatible as
used in arch/arm64/boot/dts/freescale/fsl-ls1028a.dtsi as well as the
little-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/20220425140433.33936-1-michael@walle.cc