dt-bindings: mfd: samsung,s2mps11: Convert to dtschema

Convert the MFD part of Samsung S2MPS11/S2MPS13/S2MPS14/S2MPS15/S2MPU02
family of PMICs to DT schema format.  Previously the bindings were
mostly in mfd/samsung,sec-core.txt.

The conversion copies parts of description from existing bindings
therefore the license is not changed from GPLv2.

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/20211008113931.134847-4-krzysztof.kozlowski@canonical.com