dt-bindings: net: micrel-ksz90x1.txt: correct documentation

Correct the Micrel phy documentation for the ksz9021 and ksz9031 phys
for how the phy skews are set.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>