ARM: imx: Disable 1Gbps support on MCCMON6's KSZ9031 PHY

mccmon6 works in 10/100 MiB Ethernet environment, so disabling 1GiB support
improves robustness of the network after power up (as one don't need to
wait for autoneg).

Signed-off-by: Lukasz Majewski <lukma@denx.de>