tests: drivers: uart: uart_pm: Limit scope for nrf54l15

Some configurations which are not valid for nrf54l15. Mainly those
which are using ENHANCED_POLL_OUT option which is not applicable for
nrf54l15 because it has ENDTX_STOPTX short so PPI connection is not
needed.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>