clk: max77686: Improve Maxim 77686 PMIC clocks binding

Like most clock drivers, the Maxim 77686 PMIC clock binding
follows the convention that the "#clock-cells" property is
used to specify the number of cells in a clock provider.

But the binding document is not clear enough that it shall
be set to 1 since the PMIC support multiple clocks outputs.

Also, explain that the clocks identifiers are defined in a
header file that can be included by Device Tree source with
client nodes to avoid using magic numbers.

Finally, add "clock-output-names" as an optional property
since now is supported by the clock driver.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>