scripts/dts/gen_defines.py: Fix generation for multiple PWMs

If there is more than one PWM than generate a define with a trailing
index for the PWM.  This matches what we do for GPIOs.

So something like:
  DT_PWM_LEDS_RED_PWM_LED_PWMS_CONTROLLER_0
  DT_PWM_LEDS_RED_PWM_LED_PWMS_CONTROLLER_1
  ...
  DT_PWM_LEDS_RED_PWM_LED_PWMS_CONTROLLER_<N>

Fixes #18171

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>