Add Generic subtarget definition. This is needed to keep consistent name
on every other target/subtarget and also to permit correct work of CI
workflows that expect a target/subtarget pattern.
Fixes: c16b2293fe ("ixp4xx: Resurrect IXP4xx support using device tree")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
682f8fe7e7