Doc: Micrel-ksz90x1.txt: Document deprecated MAC OF properties

Phy properties are expected to be found in the PHY OF node. However
this Micrel driver also allows them to be placed into the MAC OF node.
This is deprecated. Document it as such, and remove the example using
the deprecated method to prevent people copying it into new device
tree files.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>