arm64: dts: uniphier: fix W=2 build warnings

Fix warnings like follows:

Warning (node_name_chars_strict): Character '_' not recommended in ...

Commit 8654cb8d ("dtc: update warning settings for new bus and
node/property name checks") says these checks are a bit subjective,
but Rob also says to not add new W=2 warnings.

The exising warnings should be fixed in order to catch new ones
easily.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

arch/arm64/boot/dts/socionext/uniphier-ld11.dtsi

@@ -49,7 +49,7 @@
 		};
 	};
 
-	cluster0_opp: opp_table {
+	cluster0_opp: opp-table {
 		compatible = "operating-points-v2";
 		opp-shared;
 

arch/arm64/boot/dts/socionext/uniphier-ld20.dtsi

@@ -80,7 +80,7 @@
 		};
 	};
 
-	cluster0_opp: opp_table0 {
+	cluster0_opp: opp-table0 {
 		compatible = "operating-points-v2";
 		opp-shared;
 
@@ -118,7 +118,7 @@
 		};
 	};
 
-	cluster1_opp: opp_table1 {
+	cluster1_opp: opp-table1 {
 		compatible = "operating-points-v2";
 		opp-shared;
 

arch/arm64/boot/dts/socionext/uniphier-pxs3.dtsi

@@ -73,7 +73,7 @@
 		};
 	};
 
-	cluster0_opp: opp_table {
+	cluster0_opp: opp-table {
 		compatible = "operating-points-v2";
 		opp-shared;