stmmac: intel: add phy-mode and fixed-link ACPI _DSD setting support

Currently, phy_interface for TSN controller instance is set based on its
PCI Device ID. For SGMII PHY interface, phy_interface default to
PHY_INTERFACE_MODE_SGMII. As C37 AN supports both SGMII and 1000BASE-X
mode, we add support for 'phy-mode' ACPI _DSD for port-specific
and customer platform specific customization.

v3: use fwnode_get_phy_mode() as suggested by Andrew Lunn in
https://patchwork.kernel.org/comment/24895330/

v2:
For platform that sets 'fixed-link' using ACPI _DSD, we will unset
xpcs_an_inband within stmmac. Thanks to Russell King for his comment in
https://patchwork.kernel.org/comment/24890222/

v1:
Thanks to Andrew Lunn's guidance in
https://patchwork.kernel.org/comment/24827101/

Tested-by: Emilio Riva <emilio.riva@ericsson.com>
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>