dt-bindings: mfd: max77620: Add system-power-controller property

Document new generic property that designates the PMIC as the system's
power controller.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>