regulator: s2mps11: Implement set_voltage_time_sel() ops for bucks

Currently driver uses local struct s2mps11_info to store ramp rate for bucks
whic its getting through platform data, so instead of using regulator
constraints it should use s2mps11_info to calculate ramp delay.

Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>