Add FPUParams.fLen option, decoupled from xLen

src/main/scala/rocket/CSR.scala

@@ -333,8 +333,8 @@ class CSRFile(perfEventSets: EventSets = new EventSets(Seq()))(implicit p: Param
   val isaMaskString =
     (if (usingMulDiv) "M" else "") +
     (if (usingAtomics) "A" else "") +
-    (if (usingFPU) "F" else "") +
-    (if (usingFPU && xLen > 32) "D" else "") +
+    (if (fLen >= 32) "F" else "") +
+    (if (fLen >= 64) "D" else "") +
     (if (usingCompressed) "C" else "") +
     (if (usingRoCC) "X" else "")
   val isaString = "I" + isaMaskString +