MAINTAINERS: Add Vladimir as a maintainer for DSA

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>