net: phy: micrel: add documentation for Micrel KSZ90x1 binding

net: phy: micrel: add documentation for Micrel KSZ90x1 binding

Add the DTS documentation for the Micrel KSZ90x1 binding.

The original document was from:
[commit 4b405efbe12de28b26289282b431323d73992381 from the Linux kernel]

This takes the original document and adds a clarification on how the skew
values are represented in the code.

References:

Micrel ksz9021rl/rn Data Sheet, Revision 1.2. Dated 2/13/2014.
http://www.micrel.com/_PDF/Ethernet/datasheets/ksz9021rl-rn_ds.pdf

Micrel ksz9031rnx Data Sheet, Revision 2.1. Dated 11/20/2014.
http://www.micrel.com/_PDF/Ethernet/datasheets/KSZ9031RNX.pdf

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>