MAINTAINERS: Add Device Tree binding doc for max77686 regulator

The Device Tree binding documentation for the Maxim max77686 regulators
has been moved from the Multi-Function Device DT binding section to its
own Documentation/devicetree/bindings/regulator/max77686.txt file.

Use a wilcard so both the mfd and regulator DT bindings are resolved.

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>