Bluetooth: controller: Use PPI indices 14/15 for PA/LNA

PA/LNA feature being not default enabled, keeping the PPI
indices used for this feature at the higher indices would
allow use of these PPI indices by other drivers, for
instance the nRF5 software PWM driver.

Software PWM driver provides 3 PWM pins/channels using upto
6 PPI channels. If BLE controler where to use 0-13, then
14-19 PPI indices can be used by the PWM driver.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>