net: ethernet: ti: am65-cpsw: Enable SGMII mode for J721E

TI's J721E SoC supports SGMII mode with the CPSW9G instance of the CPSW
Ethernet Switch. Thus, enable it by adding SGMII mode to the extra_modes
member of the "j721e_cpswxg_pdata" SoC data.

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