dt-bindings: mfd: samsung,s5m8767: Document buck and LDO supplies

Document the properties with regulator supplies for bucks and LDOs.  At
least one board uses it (Exynos5250 Arndale).

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/20211008114024.135335-1-krzysztof.kozlowski@canonical.com