net: phy: add support for Micrel's KSZ9021

Add the gigabit phy KSZ9021.
Also, add function ksz9021_phy_extended_write
/_read for access to the phys extended registers.
The environment variable "disable_giga"
can be used to disable 1000baseTx.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>