Restrict clock promotion to global

ecp5/globals.cc

@@ -435,6 +435,9 @@ class Ecp5GlobalRouter
         log_info("Promoting globals...\n");
         auto clocks = get_clocks();
         for (auto clock : clocks) {
+            bool is_global = bool_or_default(clock->attrs, ctx->id("ECP5_IS_GLOBAL"), true);
+            if (!is_global)
+                continue;
             log_info("    promoting clock net %s to global network\n", clock->name.c_str(ctx));
             if (is_ooc) // Don't actually do anything in OOC mode, global routing will be done in the full design
                 clock->is_global = true;