Increased clock period to 250ns 4 MHz to match Z80 and avoid setup violations

Author: rejunity

src/config.json

@@ -17,7 +17,7 @@
 
   "//": "CLOCK_PERIOD - Increase this in case you are getting setup time violations.",
   "//": "The value is in nanoseconds, so 20ns == 50MHz.",
-  "CLOCK_PERIOD": 20,
+  "CLOCK_PERIOD": 250,
 
   "//": "Hold slack margin - Increase them in case you are getting hold violations.",
   "PL_RESIZER_HOLD_SLACK_MARGIN": 0.1,