dts: arm: nordic: Add support for ieee802154 in the nRF52820 radio

The nRF52820 radio peripheral supports IEEE 802.15.4, add the required
property and node to reflect this.

See
https://infocenter.nordicsemi.com/topic/ps_nrf52820/radio.html?cp=5_3_0_5_11



Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>