net: ethernet: ti: am65-cpsw: Enable QSGMII for J784S4 CPSW9G

TI's J784S4 SoC supports QSGMII mode with the CPSW9G instance of the
CPSW Ethernet Switch. Add a new compatible for J784S4 SoC and enable
QSGMII support for it by adding QSGMII mode to the extra_modes member of
the "j784s4_cpswxg_pdata" SoC data.

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