tools: ynl-gen: track attribute use

For range validation we'll need to know if any individual
attribute is used on input (i.e. whether we will generate
a policy for it). Track this information.

Link: https://lore.kernel.org/r/20231018163917.2514503-2-kuba@kernel.org


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