arm: socfpga: misc: Probe ethernet GMAC from OF

The GMAC can now be probed from OF, so enable DM ethernet and remove the
old ad-hoc designware_initialize() invocation.

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