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 <enric.balletbo@collabora.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>

Documentation/devicetree/bindings/devfreq/rk3399_dmc.txt

@@ -5,10 +5,6 @@ Required properties:
 - 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";
@@ -17,6 +13,12 @@ Required properties:
 - 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:
 
 - rockchip,dram_speed_bin :	  Value reference include/dt-bindings/clock/rk3399-ddr.h,