drivers: net: cpsw: use cpsw-phy-sel driver to configure phy mode

Phy mode can be configured via the cpsw-phy-sel driver, this patch enabled the
cpsw driver to utilise the api provided by the cpsw-phy-sel driver to configure
the phy mode.

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