tools: ynl-gen: allow noncontiguous enums

in case the enum has holes, instead of hard stop, generate a validation
callback to check valid enum values.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://patch.msgid.link/20250505114513.53370-2-jiri@resnulli.us


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