net: dwc_eth_xgmac_socfpga: Add support for distinct mac-mode and phy mode.

net: dwc_eth_xgmac_socfpga: Add support for distinct mac-mode and phy mode.

This patch adds support for configuring the ethernet MAC mode independently
from the PHY mode on our SoC FPGA board. Specifically, this is necessary
for a scenario where the ethernet controller MAC is connected to the
FPGA HVIO with a different GMII interface, and the FPGA output is routed
to the PHY using a RGMII interface.

To support this configuration, a mechanism is introduced to handle
separate MAC mode settings, ensuring that the MAC controller and PHY
can operate correctly with their respective interface modes.

If mac-mode is not defined, the MAC mode will default to the PHY mode,
ensuring compatibility and proper operation between the MAC and PHY.

Signed-off-by: Boon Khai Ng <boon.khai.ng@intel.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@intel.com>