net: ieee802154_radio: add attribute getter API

The `attr_get` method is added to the ieee802154_radio to allow
reading of driver specific attributes of given device.

The enum `ieee802154_attr` provides common extension pattern
allowing to extend the attribute set.

Accessor function `ieee802154_radio_attr_get` is provided.

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