mfd: max77686: Split out regulator part from the DT binding

The Maxim MAX77686 PMIC is a multi-function device with regulators,
clocks and a RTC. The DT bindings for the clocks are in a separate
file but the bindings for the regulators are inside the mfd part.

To make it consistent with the clocks portion of the binding and
because is more natural to look for regulator bindings under the
bindings/regulator sub-directory, split the regulator portion of
the DT binding and add it as a separate file.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>