Update Micrel KSZ90x1 binding documentation

Renames micrel-ksz9021.txt to micrel-ksz90x1.txt and adds documentation for
the KSZ9031 binding from patch 1. Also adds step increment information, and
note about phy fixups.

Signed-off-by: Hubert Chaumette <hchaumette@adeneo-embedded.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>