regulator: max77686: Implement .set_ramp_delay() callback

This patch implements the .set_ramp_delay callback to set the ramp_delay on
hardware for BUCK2/3/4 if ramp_delay is set in regulator constraints.

This patch also do some cleaning work for unrequired members of
struct max77686_data.

Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>