regulator: max77693: Add max77693 regualtor driver.

This patch adds new regulator driver to support max77693 chip's regulators.
max77693 has two linear voltage regulators and one current regulator which
can be controlled through I2C bus. This driver also supports device tree.

Signed-off-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>