dt-bindings: mfd: s2mps11: Add support S2MPU02 PMIC

This patch add documentation for S2MPU02 PMIC device. S2MPU02 has a little
difference from S2MPS11/S2MPS14 PMIC and has LDO[1-28]/Buck[1-7].

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>