Transceiver52M: Add option to explicitly set GPSDO reference

Before UHD version 3.9.0 on-board GPSDO modules for USRP
devices were automatically enabled if detected. After UHD
version 3.9.0 the UHD behavior changed to require explicit
setting of the reference to enable GPSDO use.

Modify the interfaces to pass 'internal', 'external' and
'gpsdo' reference options from configuration database into
the UHD driver layer. Convert the 'TRX.reference' entry in
the configuration from a boolean value to string.

Transceiver52M/USRPDevice.h

@@ -105,7 +105,7 @@ private:
   USRPDevice(int sps, bool skipRx);
 
   /** Instantiate the USRP */
-  int open(const std::string &, bool);
+  int open(const std::string &, ReferenceType);
 
   /** Start the USRP */
   bool start();