Fix warnings like follows:
Warning (node_name_chars_strict): Character '_' not recommended in ...
Commit 
8654cb8d0371 ("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>
                };
        };
 
-       cluster0_opp: opp_table {
+       cluster0_opp: opp-table {
                compatible = "operating-points-v2";
                opp-shared;
 
 
                };
        };
 
-       cluster0_opp: opp_table0 {
+       cluster0_opp: opp-table0 {
                compatible = "operating-points-v2";
                opp-shared;
 
                };
        };
 
-       cluster1_opp: opp_table1 {
+       cluster1_opp: opp-table1 {
                compatible = "operating-points-v2";
                opp-shared;
 
 
                };
        };
 
-       cluster0_opp: opp_table {
+       cluster0_opp: opp-table {
                compatible = "operating-points-v2";
                opp-shared;