arm64: dts: zte: Use - instead of @ for DT OPP entries

Compiling the DT file with W=1, DTC warns like follows:

Warning (unit_address_vs_reg): Node /opp_table0/opp@1000000000 has a
unit name, but no reg property

Fix this by replacing '@' with '-' as the OPP nodes will never have a
"reg" property.
Reported-by: NKrzysztof Kozlowski <krzk@kernel.org>
Reported-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Suggested-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NViresh Kumar <viresh.kumar@linaro.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/zte/zx296718.dtsi

@@ -118,27 +118,27 @@
 		compatible = "operating-points-v2";
 		opp-shared;
 
-		opp@500000000 {
+		opp-500000000 {
 			opp-hz = /bits/ 64 <500000000>;
 			clock-latency-ns = <500000>;
 		};
 
-		opp@648000000 {
+		opp-648000000 {
 			opp-hz = /bits/ 64 <648000000>;
 			clock-latency-ns = <500000>;
 		};
 
-		opp@800000000 {
+		opp-800000000 {
 			opp-hz = /bits/ 64 <800000000>;
 			clock-latency-ns = <500000>;
 		};
 
-		opp@1000000000 {
+		opp-1000000000 {
 			opp-hz = /bits/ 64 <1000000000>;
 			clock-latency-ns = <500000>;
 		};
 
-		opp@1188000000 {
+		opp-1188000000 {
 			opp-hz = /bits/ 64 <1188000000>;
 			clock-latency-ns = <500000>;
 		};