ecp5: Add attrmvcp to copy syn_useioff to driving FF

Signed-off-by: David Shah <dave@ds0.me>

techlibs/ecp5/synth_ecp5.cc

@@ -297,6 +297,7 @@ struct SynthEcp5Pass : public ScriptPass
 			run("simplemap");
 			run("ecp5_ffinit");
 			run("ecp5_gsr");
+			run("attrmvcp -copy -attr syn_useioff");
 			run("opt_clean");
 		}