drivers: ieee802154_nrf5: Use BLE.ADDR instead of FICR to create EUI64.

Use BLE.ADDR to create unique (to some extent) EUI64
on nRF54H20 in some cases inside the IEEE 802.15.4 driver.

The amount of EUI64-s available in such a way is very limited
(~16 million).
However, currently there does not seem to be another feasible way to
get device identifiers on nRF54H20 (such are kept in SICR, to which the
radio core has no access).

Signed-off-by: Piotr Koziar <piotr.koziar@nordicsemi.no>