regulator: max77650: use vsel_step

Use the new vsel_step field in the regulator description to instruct
the regulator API on the required voltage ramping. Switch to using the
generic regmap helpers for voltage setting and remove the old set_voltage
callback that handcoded the selector stepping.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20190703161035.31808-3-brgl@bgdev.pl
Signed-off-by: Mark Brown <broonie@kernel.org>