net: phy: micrel: make ksz9021 phy accessible

Micrel accidentally used the same part number
for the KS8721 and KSZ9021. So, both cannot be
in the same build of u-boot. Add a config option
to handle this.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>