net: dsa: mv88e6xxx: Add serdes interrupt support for MV88E6097

The MV88E6097 presents the serdes interrupts for ports 8 and 9 via the
Switch Global 2 registers. There is no additional layer of
enablinh/disabling the serdes interrupts like other mv88e6xxx switches.
Even though most of the serdes behaviour is the same as the MV88E6185
that chip does not provide interrupts for serdes events so unlike
earlier commits the functions added here are specific to the MV88E6097.

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>