dt-bindings: mfd: samsung,s2mpa01: Convert to dtschema

Convert the MFD part of Samsung S2MPA01 PMIC to DT schema format.
Previously the bindings were mostly in mfd/samsung,sec-core.txt.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211008113931.134847-5-krzysztof.kozlowski@canonical.com