[nrf fromlist] boards: nordic: nrf54l20pdk: Add PWM to the list of supported peripherals

Enable Twister tests of PWM driver on nrf54l20pdk.
Overlays were already added to PWM tests.

Upstream PR #: 85721

Signed-off-by: Sebastian Głąb <[email protected]>

Author: nordic-segl

boards/nordic/nrf54l20pdk/nrf54l20pdk_nrf54l20_cpuapp.yaml

@@ -16,4 +16,5 @@ supported:
   - counter
   - gpio
   - i2c
+  - pwm
   - watchdog