regulator: s2mps11: Implement set_ramp_rate callback for bucks

Implementing set_ramp_rate() and using standard constraints for getting
ramp_delay and ramp_disable, instead of getting it as s2mps11 specific data
through platform data, makes driver more compliant with framework and reduces
the complexity for adding DT support.

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