net: dsa: mv88e6xxx: 88E6141/6341 SERDES support

The 88E6141/6341 switches (also known as Topaz) have 1 SGMII lane,
which can be configured the same way as the SERDES lane on 88E6390.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>