net: ti cpsw ethernet: allow reading phy interface mode from DT

Allow users to specify the phy interface of the CPSW slaves. The new
node parameter is called "phy_if_mode" and is optional. The original
behaviour of the driver is preserved when not given.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
Cc: Vaibhav Hiremath <hvaibhav@ti.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>