regulator: add device tree support for s5m8767

This device tree support is added for PMIC block of S5m8767 multi
function driver. The usage detail is added in the device tree
documentation section. This change is tested on exynos5250 based
arndale platform by regulator voltage set/get API's.

Reviewed-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Tested-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>