ARM: dts: rockchip: Fix gpu opp node names for rk3288

The device tree compiler yells like this:
  Warning (unit_address_vs_reg):
  /gpu-opp-table/opp@100000000:
  node has a unit name, but no reg property

Let's match the cpu opp node names and use a dash.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

arch/arm/boot/dts/rk3288.dtsi

@@ -1282,27 +1282,27 @@ gpu: gpu@ffa30000 {
 	gpu_opp_table: gpu-opp-table {
 		compatible = "operating-points-v2";
 
-		opp@100000000 {
+		opp-100000000 {
 			opp-hz = /bits/ 64 <100000000>;
 			opp-microvolt = <950000>;
 		};
-		opp@200000000 {
+		opp-200000000 {
 			opp-hz = /bits/ 64 <200000000>;
 			opp-microvolt = <950000>;
 		};
-		opp@300000000 {
+		opp-300000000 {
 			opp-hz = /bits/ 64 <300000000>;
 			opp-microvolt = <1000000>;
 		};
-		opp@400000000 {
+		opp-400000000 {
 			opp-hz = /bits/ 64 <400000000>;
 			opp-microvolt = <1100000>;
 		};
-		opp@500000000 {
+		opp-500000000 {
 			opp-hz = /bits/ 64 <500000000>;
 			opp-microvolt = <1200000>;
 		};
-		opp@600000000 {
+		opp-600000000 {
 			opp-hz = /bits/ 64 <600000000>;
 			opp-microvolt = <1250000>;
 		};