mfd: max77686: Use a generic name for the PMIC node in the example

The ePAPR standard says that: "the name of a node should be somewhat
generic, reflecting the function of the device and not its precise
programming model."

So, change the max77686 binding document example to use a generic
node name instead of using the chip's name.

Suggested-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>