dt-bindings: devfreq: rk3399_dmc: fix clocks in example

The clocks in the example use 'SCLK_DDRCLK', which does not exist in
the rk3399-cru.h, correct it to 'SCLK_DDRC'.

Ref: commit 7fbdfcd6 ("clk: rockchip: add SCLK_DDRC id for rk3399 ddrc")
Fixes: c1ceb8f7 ("Documentation: bindings: add dt documentation for rk3399 dmc")
Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Link: https://lore.kernel.org/r/20211001150024.18672-1-amadeus@jmu.edu.cnSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/devfreq/rk3399_dmc.txt

@@ -174,7 +174,7 @@ Example:
 		compatible = "rockchip,rk3399-dmc";
 		devfreq-events = <&dfi>;
 		interrupts = <GIC_SPI 1 IRQ_TYPE_LEVEL_HIGH>;
-		clocks = <&cru SCLK_DDRCLK>;
+		clocks = <&cru SCLK_DDRC>;
 		clock-names = "dmc_clk";
 		operating-points-v2 = <&dmc_opp_table>;
 		center-supply = <&ppvar_centerlogic>;