dts: Added dwcxgmac dt nodes in soc dtsi file

Added XGMAC0, XGMAC1, XGMAC2 device nodes in
intel_socfpga_agilex5 dts file with default
parameter values and  default device node status
as 'disabled'.

Signed-off-by: Santosh Male <santosh.male@intel.com>