tools: ynl: generate code for the devlink family

Admittedly the devlink.yaml spec is fairly limitted,
it only covers basic device get and info-get ops.
That's sufficient to be useful (monitoring FW versions
in the fleet). Plus it gives us a chance to exercise
deep nesting and directional messaging in YNL.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>