drivers:power:max77686: add function to set voltage and mode

This patch add new functions to pmic max77686 to set voltage and mode.

Signed-off-by: Piotr Wilczek <p.wilczek@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Rajeshwari Shinde <rajeshwari.s@samsung.com>
Acked-by: Tom Rini <trini@ti.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>