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>