dt-bindings: Add MP2856/MP2857 voltage regulator device

Monolithic Power Systems, Inc. (MPS) MP2856/MP2857
dual-loop, digital, multi-phase controller.

Signed-off-by: Peter Yin <peteryin.openbmc@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231211160519.21254-2-potin.lai.pt@gmail.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>