net/mlx5: Document flow_steering_mode devlink param

Add documentation for current mlx5 supported devlink param.

Signed-off-by: Michael Guralnik <[e-mail]>
Acked-by: Jiri Pirko <[e-mail]>
Signed-off-by: Saeed Mahameed <[e-mail]>

Documentation/networking/devlink-params-mlx5.txt

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+flow_steering_mode	[DEVICE, DRIVER-SPECIFIC]
+			Controls the flow steering mode of the driver.
+			Two modes are supported:
+			1. 'dmfs' - Device managed flow steering.
+			2. 'smfs  - Software/Driver managed flow steering.
+			In DMFS mode, the HW steering entities are created and
+			managed through the Firmware.
+			In SMFS mode, the HW steering entities are created and
+			managed though by the driver directly into Hardware
+			without firmware intervention.
+			Type: String
+			Configuration mode: runtime