drivers: pwm: Fix prescaler configuration

This commit fixes the bug in the prescaler configuration formula.
Prescaler enumarations values are not consecutive numbers, so this
formula does not work correctly. This commit adds prescaler enums into
an array and sets PWM prescaler from this array with using
prescaler_index parameter.

Signed-off-by: Furkan Akkiz <hasanfurkan.akkiz@analog.com>