arm: socfpga: socrates: Add missing PHY skew config

Add missing KSZ9021 PHY skew configuration for the EBV socrates board.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>