ynl: support enum-cnt-name attribute in legacy definitions

This is similar to existing attr-cnt-name in the attributes
to allow changing the name of the 'count' enum entry.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241204155549.641348-2-sdf@fomichev.me


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