dt-bindings: net: ethernet-controller: add 5000M speed to fixed-link

A link speed of 5000 Mbps is a valid speed for a fixed-link mode of
operation. Hence, update the bindings to include the same.

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250411060917.633769-2-s-vadapalli@ti.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>