Change default LF clock source for LP_CC2652RB board to RCOSC_LF

The default LF clock source for all devices and boards is currently kept as "LF XOSC". 
This is correct in almost all cases with the exception of the LP_2652RB launchpad where the board is not shipped with an LF crystal by default.
One of the consequences of this, is that the device is prevented from entering standby mode. 
This can be tested by executing the empty example with the following modifications:

• Register for power notification for event PowerCC26XX_ENTERING_STANDBY. and then force the device to sleep with a sleep(1000). 
• In the notification callback, drive a GPIO pin and capture this on the logic analyzer. 

We observe that with the LP_2652RB launchpad and the clock source being LF XOSC, the device never triggers this notification. When changed to LF RCOSC, the device triggers the notification. 

Thus, the suggested default clock source only for this launchpad could be LF_RCOSC.