samples: basic: fix nRF52 PWM name in blink_led/fade_led samples

commit ddf744deee39 ("drivers: pwm_nrfx: Use Device Tree label as
instance name") removed the config setting for PWM names and replaced
it with DT_NORDIC_NRF_PWM_PWM_*_LABEL.

This change is causing the blink_led and fade_led samples to break
when compiled for nRF52 boards.

Let's update the samples to use the DT setting.

Signed-off-by: Michael Scott <mike@foundries.io>
2 files changed