net: ethernet: ti: am65-cpsw: Enable USXGMII mode for J784S4 CPSW9G

TI's J784S4 SoC supports USXGMII mode. Add USXGMII mode to the
extra_modes member of the J784S4 SoC data.

Configure MAC control register for supporting USXGMII mode and add
MAC_5000FD in the "mac_capabilities" member of struct "phylink_config".

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