tests: drivers: audio: dmic_api: Enable test execution on nrf54l15

Enable execution of dmic_api test on nrf54l15:
- add 'dmic' to the list of supported peripherals;
- add overlay for nrf54l15;
- align Nordic's implementation of PDM driver to pass the test.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>