Bluetooth: Controller: Use configured clock source

Remove hardcoded use of crystal as 32KHz clock source and
20 ppm accuracy; and use the values from config for sleep
clock source and sleep clock accuracy value in Bluetooth
Controller.

Change-id: I1c0d53ecf8ad158153d5186a6680b5eb03d1641b
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>