gpu: change the maximal frequency to 600Mhz for GPU.

Change-Id: I29847de99f2bfe58802ed46fd9126aca4a054a3b

Author: singhomelee

arch/arm/boot/dts/rk3288.dtsi

@@ -1795,9 +1795,9 @@
 			opp-hz = /bits/ 64 <400000000>;
 			opp-microvolt = <1100000>;
 		};
-		opp-500000000 {
-			opp-hz = /bits/ 64 <500000000>;
-			opp-microvolt = <1200000>;
+		opp-600000000 {
+			opp-hz = /bits/ 64 <600000000>;
+			opp-microvolt = <1250000>;
 		};
 	};