dt-bindings: mfd: samsung,s5m8767: Convert to dtschema

Convert the MFD part of Samsung S5M8767 PMIC to DT schema format.
Previously the bindings were mostly in mfd/samsung,sec-core.txt.

Since all of bindings for Samsung S2M and S5M family of PMICs were
converted from mfd/samsung,sec-core.txt to respective dtschema file,
remove the former one.

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/20211008113931.134847-6-krzysztof.kozlowski@canonical.com