MAINTAINERS: add an explicit entry for YNL

Donald has been contributing to YNL a lot. Let's create a dedicated
MAINTAINERS entry and add make his involvement official :)

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>