drivers: pwm: nRF5x: Use TIMER2 and PPI indices 14-19

As the nRF5x BLE controller uses TIMER1, TIMER2 and PPI
indices 0-13 (and 14-15, if PA/LNA feature is enabled),
the software based PWM driver needing 6 PPI channels has
to use PPI indices outside the BLE controller used range.

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