[sim] Use DutyCycleEncoder FPGA index for sim device number (#2803)

The source channel doesn't necessarily correspond with the HALSIM device
index.

wpilibc/src/main/native/include/frc/DutyCycleEncoder.h

@@ -154,6 +154,13 @@ class DutyCycleEncoder : public ErrorBase,
    */
   double GetDistance() const;
 
+  /**
+   * Get the FPGA index for the DutyCycleEncoder.
+   *
+   * @return the FPGA index
+   */
+  int GetFPGAIndex() const;
+
   /**
    * Get the channel of the source.
    *