zephyr-openthread-rcp: Support nrf52840dk-nrf52840 machine

This Nordic reference design can be a fallback option for Oniro's gateway-blueprint I've been tested it myself along my single arduino-nano node

Forwarded: https://gitlab.eclipse.org/eclipse/oniro-core/meta-zephyr//pulls?q=author%3Arzr Relate-to: eclipse/oniro-blueprints/transparent-gateway/meta-oniro-blueprints-gateway#6 Signed-off-by: Philippe Coval philippe.coval.ext@huawei.com