net: designware: Add SoCFPGA GMAC DT compatible string

Add the OF compatible property to match the SoCFPGA GMAC.

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