arm: socfpga: de0_nano: Remove Micrel PHY configuration

The Micrel PHY configuration is now done from OF, so hard-coding
the configuration into the board file is no longer necessary.

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>