devicetree: net: micrel-ksz90x1.txt: Properly explain skew settings

The KSZ9031 skew registers contain an offset, the chip's default value
is "neutral" which does not add any skew. Programming a 0 into a skew
property will actually set it the maximal negative adjustment and not
to a neutral position as one would expect.

Explain this situation in the devicetree binding documentation and list
the settings that the chip considers neutral.

Changing the implementation to accept negative values would have been
a better solution, but would break existing configurations.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/micrel-ksz90x1.txt

@@ -34,16 +34,17 @@ KSZ9031:
 
   All skew control options are specified in picoseconds. The minimum
   value is 0, and the maximum is property-dependent. The increment
-  step is 60ps.
+  step is 60ps. The default value is the neutral setting, so setting
+  rxc-skew-ps=<0> actually results in -900 picoseconds adjustment.
 
   Optional properties:
 
-    Maximum value of 1860:
+    Maximum value of 1860, default value 900:
 
       - rxc-skew-ps : Skew control of RX clock pad
       - txc-skew-ps : Skew control of TX clock pad
 
-    Maximum value of 900:
+    Maximum value of 900, default value 420:
 
       - rxdv-skew-ps : Skew control of RX CTL pad
       - txen-skew-ps : Skew control of TX CTL pad