boards: nrf54l15pdk_nrf54l15_cpuapp: add ieee802154 status okay

Set status = "okay" for ieee802154 device tree node to allow use
of ieee802154 on the nrf54l15pdk_nrf54l15_cpuapp board.

Signed-off-by: Andrzej Kuroś <andrzej.kuros@nordicsemi.no>