net: dsa: mv88x6xxx: mv88e6390 errata

[ Upstream commit ea89098ef9a574bceca00d3b5df14aaf0b3f9ccf ]

The 6390 copper ports have an errata which require poking magic values
into undocumented magic registers and then performing a software
reset.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>