arm: socfpga: stratix10: Add alias for gmac0 in S10 dts

Add 'ethernet0' as alias for 'gmac0' in S10 device tree.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Signed-off-by: Ooi, Joyce <joyce.ooi@intel.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>