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: !22
Relate-to: eclipse/oniro-blueprints/transparent-gateway/meta-oniro-blueprints-gateway#6


Signed-off-by: Philippe Coval <philippe.coval.ext@huawei.com>