net: dsa: microchip: Add PHY errata workarounds

The Silicon Errata and Data Sheet Clarification documents for the
KSZ9477 series of chips describe a number of otherwise undocumented PHY
register settings which are required to work around various chip errata.
Apply these settings when initializing the PHY ports on these chips.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>