mx6sabresd: Make Ethernet functional again

mx6sabresd: Make Ethernet functional again

Since commit ce412b79e7255770 ("drivers: net: phy: atheros: add separate
config for AR8031") ethernet does not work on mx6sabresd.

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

However, on mx6sabresd design we need some additional configuration,
such as enabling the 125 MHz AR8031 output that needs to be done
in the board file.

This also aligns with the same method that the kernel performs
the AR8031 fixup in arch/arm/mach-imx/mach-imx6q.c.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>