mx6sabreauto: Make Ethernet functional again

mx6sabreauto: Make Ethernet functional again

Since commit ce412b79e7255770 ("drivers: net: phy: atheros: add separate
config for AR8031") Ethernet does not work on mx6sabreauto.

This commit correctly assigns ar8031_config() as the configuration
function for AR8031 in the same way as done in the Linux kernel.

However, on mx6sabreauto design we need some additional configurations,
such as enabling the 125 MHz AR8031 output and setting the TX clock
delay that need to be done in the board file.

This is the equivalent fix from commit 4b6035da482c ("mx6sabresd: Make
Ethernet functional again").

Reported-by: Miquel RAYNAL <miquel.raynal@free-electrons.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>