arm: socfpga: misc: Reset ethernet from OF

Reset the GMAC ethernets based on the "resets" OF node instead of ad-hoc
hardcoded values in the U-Boot code. Since we don't have a proper reset
framework in place yet, we have to do this slightly ad-hoc parsing of the
OF tree instead.

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