MAINTAINERS: add myself as maintainer for enetc

I would like to be copied on new patches submitted on this driver.
I am relatively familiar with the code, having practically maintained
it for a while.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>