net: phy: dp83869: Add speed optimization feature

Set the speed optimization bit on the DP83869 PHY.

Speed optimization, also known as link downshift, enables fallback to 100M
operation after multiple consecutive failed attempts at Gigabit link
establishment. Such a case could occur if cabling with only four wires
(two twisted pairs) were connected instead of the standard cabling with
eight wires (four twisted pairs).

The number of failed link attempts before falling back to 100M operation is
configurable. By default, four failed link attempts are required before
falling back to 100M.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>