net/phy: micrel: Add OF configuration support for ksz9021

Some boards require custom PHY configuration, for example due to trace
length differences.  Add the ability to configure these registers in
order to get the PHY to function on boards that need it.

Because PHYs are auto-detected based on MDIO device IDs, allow PHY
configuration to be specified in the parent Ethernet device node if no
PHY device node is present.

Signed-off-by: Sean Cross <xobs@kosagi.com>
Signed-off-by: David S. Miller <davem@davemloft.net>