net: dsa: mv88e6xxx: fully support SERDES on Topaz family

Currently we support SERDES on the Topaz family in a limited way: no
IRQs and the cmode is not writable, thus the mode is determined by
strapping pins.

Marvell's examples though show how to make cmode writable on port 5 and
support SGMII autonegotiation. It is done by writing hidden registers,
for which we already have code.

This patch adds support for making the cmode for the SERDES port
writable on the Topaz family, via a new chip operation,
.port_set_cmode_writable, which is called from mv88e6xxx_port_setup_mac
just before .port_set_cmode.

SERDES IRQs are also enabled for Topaz.

Tested on Turris Mox.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>