regulator: max77686: Add device tree support.

This patch device tree support for regulator driver. It uses the parent
(mfd's) DT node to parse the regulator data for max77686.

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