zephyr-openthread-rcp: Enable Thread 1.2 capabilities in RCP build

When running an up-to-date userspace component, like otbr-agent or
ot-daemon, with Thread 1.2 enabled we need a firmware matching this
capabilities.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>
Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>