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 <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/max77620.txt

···
 			IRQ numbers for different interrupt source of MAX77620
 			are defined at dt-bindings/mfd/max77620.h.
 
+- system-power-controller: Indicates that this PMIC is controlling the
+			   system power, see [1] for more details.
+
+[1] Documentation/devicetree/bindings/power/power-controller.txt
+
 Optional subnodes and their properties:
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