Make it easier to override the 'placeholder' Real-Time-Clock, to allow more real-world applications

src/main/scala/rocketchip/Configs.scala

@@ -153,6 +153,8 @@ class BasePlatformConfig extends Config (
       case ExtMemSize => Dump("MEM_SIZE", 0x10000000L)
       case ConfigString => makeConfigString()
       case GlobalAddrMap => globalAddrMap
+      case RTCPeriod => 100 // gives 10 MHz RTC assuming 1 GHz uncore clock
+      case RTC => (p: Parameters, t_io: Bundle, p_io:Bundle) => Counter(p(RTCPeriod)).inc() 
       case _ => throw new CDEMatchError
   }})