MAINTAINERS: add entry for macsec

Jakub asked if I'd be willing to be the maintainer of the macsec code
and review the driver code adding macsec offload, so let's add the
corresponding entry.

The keyword lines are meant to catch selftests and patches adding HW
offload support to other drivers.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>