q6_0: slightly better kv-cache result

Better than q8_0+q4_0, but not as good as q8_0+iq4_nl

examples/llama-bench/llama-bench.cpp

@@ -327,6 +327,9 @@ static ggml_type ggml_type_from_name(const std::string & s) {
     if (s == "iq4_nl") {
         return GGML_TYPE_IQ4_NL;
     }
+    if (s == "q6_0") {
+        return GGML_TYPE_Q6_0;
+    }
 
     return GGML_TYPE_COUNT;
 }