Build warnings with atsamr21_xpro

[pabigot]

#22253 broke atsamr21_xpro because the board defconfig has inconsistent dependencies:

% west build -b atsamr21_xpro samples/hello_world
...
Kconfig header saved to '/mnt/nordic/zp/zephyr/build/zephyr/include/generated/autoconf.h'

warning: IEEE802154_RF2XX (defined at drivers/ieee802154/Kconfig.rf2xx:6) was assigned the value 'y'
but got the value 'n'. Check these unsatisfied dependencies: NETWORKING (=n), IEEE802154 (=n). See
http://docs.zephyrproject.org/latest/reference/kconfig/CONFIG_IEEE802154_RF2XX.html and/or look up
IEEE802154_RF2XX in the menuconfig/guiconfig interface. The Application Development Primer, Setting
Configuration Values, and Kconfig - Tips and Best Practices sections of the manual might be helpful
too.

This breaks CI for any PR that happens to trigger a build on this platform.

[carlescufi]

Fixed in master