dt-bindings: hwmon: max20730: adding device tree doc for max20730

max20730 Integrated, Step-Down Switching Regulator with PMBus

Signed-off-by: Chu Lin <linchuyuan@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201004031445.2321090-2-linchuyuan@google.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>