net: ethernet: ti: am65-cpsw: Add support for SGMII mode

Add support for configuring the CPSW Ethernet Switch in SGMII mode.

Depending on the SoC, allow selecting SGMII mode as a supported interface,
based on the compatible used.

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>