regulator: max77686: Convert driver to use regulator_set_voltage_time_sel.

This patch converts the driver to use regulator_set_voltage_time_sel() as
.set_voltage_time_sel() callback. It also sets ramp_delay as 100000 uV/us for
LDOs & normal BUCKs and 27500 uV/us(default/reset value) for BUCKs[2/3/4] in
regulator_desc[].

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