tools: ynl-gen: store recursive nests by a pointer

To avoid infinite nesting store recursive structs by pointer.
If recursive struct is placed in the op directly - the first
instance can be stored by value. That makes the code much
less of a pain for majority of practical uses.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231213231432.2944749-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>