modules: nordic: Allow separate nRF 802.15.4 radio driver build

So far, nRF 802.15.4 radio driver build was dependent on the 802.15.4
subsystem in Zephyr. While this was a reasonable approach for samples,
it prevented the radio driver from being built as a standalone entity,
which could be useful in some applications (e. g. running core nRF
802.15.4 radio driver tests with Zephyr).

Resolve this, by providing a separate set of Kconfigs for the radio
driver, therefore allowing to build it as a separate entity. The 802154
subsystem simply enables the radio driver module in this case.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>