dt-bindings: devfreq: rk3399_dmc: move interrupts to be optional.

In ATF we already wait for DDR DVFS finish, so move the interrupts properties
to be optional.

Signed-off-by: Enric Balletbo i Serra <[e-mail]>
Reviewed-by: Chanwoo Choi <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: MyungJoo Ham <[e-mail]>

Documentation/devicetree/bindings/devfreq/rk3399_dmc.txt

···
 - devfreq-events:	 Node to get DDR loading, Refer to
 			 Documentation/devicetree/bindings/devfreq/event/
 			 rockchip-dfi.txt
-- interrupts:		 The CPU interrupt number. The interrupt specifier
-			 format depends on the interrupt controller.
-			 It should be a DCF interrupt. When DDR DVFS finishes
-			 a DCF interrupt is triggered.
 - clocks:		 Phandles for clock specified in "clock-names" property
 - clock-names :		 The name of clock used by the DFI, must be
 			 "pclk_ddr_mon";
···
 			 for details.
 - center-supply:	 DMC supply node.
 - status:		 Marks the node enabled/disabled.
+
+Optional properties:
+- interrupts:		 The CPU interrupt number. The interrupt specifier
+			 format depends on the interrupt controller.
+			 It should be a DCF interrupt. When DDR DVFS finishes
+			 a DCF interrupt is triggered.
 
 Following properties relate to DDR timing: