MAINTAINERS: Add an entry for the core network DSA code

The core distributed switch architecture code currently does not have
a MAINTAINERS entry, which results in some contributions not landing
in the right peoples inbox.

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