phy: ti: gmii-sel: Enable SGMII mode for J784S4

TI's J784S4 SoC supports SGMII mode with the CPSW9G instance of the CPSW
Ethernet Switch. Thus, enable it by adding SGMII mode to the list of the
corresponding extra_modes member.

Signed-off-by: Chintan Vankar <c-vankar@ti.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Link: https://lore.kernel.org/r/20231221102956.754617-1-c-vankar@ti.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>