net: dsa: mv88e6xxx: move driver in its own folder

With the upcoming support for cross-chip operations and other mv88e6xxx
enhancements, new files will be added.

Similarly to mlxsw or b53, move mv88e6xxx files into their own folder.

In the meantime, update the MAINTAINERS entry to please checkpatch.pl,
by replacing the invalid 88E6352 entry with 88E6XXX, maintained by
Andrew and myself.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>