ARM: dts: exynos: Fix DTC unit name warnings in SMDK5420

Fix following DTC warnings in Exynos5420 SMDK5420:

Warning (unit_address_vs_reg): Node /dp-controller@145B0000/display-timings/timing@0 has a unit name, but no reg property
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>

arch/arm/boot/dts/exynos5420-smdk5420.dts

@@ -111,7 +111,7 @@ &dp {
 
 	display-timings {
 		native-mode = <&timing0>;
-		timing0: timing@0 {
+		timing0: timing {
 			clock-frequency = <50000>;
 			hactive = <2560>;
 			vactive = <1600>;