ARM: i.MX6: Add OF configuration support for ksz9031

Adds support for ksz9031 PAD skew configuration over devicetree.

Signed-off-by: Hubert Chaumette <hchaumette@adeneo-embedded.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>