mfd: max77686: Don't suggest in binding to use a deprecated property

The regulator-compatible property from the regulator DT binding was
deprecated. But the max77686 DT binding doc still suggest to use it
instead of the regulator node name's which is the correct approach.

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>