phy: ti: gmii-sel: 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 list of the
corresponding extra_modes member.

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Link: https://lore.kernel.org/r/20230309063514.398705-4-s-vadapalli@ti.com


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