net: dsa: mv88e6xxx: Support serdes ports on MV88E6097/6095/6185

Implement serdes_power, serdes_get_lane and serdes_pcs_get_state ops for
the MV88E6097/6095/6185 so that ports 8 & 9 can be supported as serdes
ports and directly connected to other network interfaces or to SFPs
without a PHY.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>