Add WithClockFrequency class to update frequencies

src/main/scala/subsystem/Configs.scala

@@ -230,6 +230,14 @@ class WithRoccExample extends Config((site, here, up) => {
     }
 })
 
+class WithClockFrequency(frequency: BigInt) extends Config((site, here, up) => {
+  case RocketTilesKey => up(RocketTilesKey, site) map { r =>
+    r.copy(core = r.core.copy(bootFreqHz = frequency))
+  }
+  case PeripheryBusKey => up(PeripheryBusKey, site)
+    .copy(frequency = frequency)
+})
+
 class WithDefaultBtb extends Config((site, here, up) => {
   case RocketTilesKey => up(RocketTilesKey, site) map { r =>
     r.copy(btb = Some(BTBParams()))