config: use Field defaults over Config defaults

Also rename some keys that had the same class name as their value's class name.

src/main/scala/diplomacy/DeviceTree.scala

@@ -8,7 +8,7 @@ import java.io.{ByteArrayInputStream, ByteArrayOutputStream}
 
 case object DTSModel    extends Field[String]
 case object DTSCompat   extends Field[Seq[String]] // -dev, -soc
-case object DTSTimebase extends Field[BigInt]      // Clock frequency of clint RTC (use 0 if you don't know it)
+case object DTSTimebase extends Field[BigInt](0)   // Clock frequency of clint RTC (use 0 if you don't know it)
 
 object DTS
 {