UBUNTU: [Config]: turn off CONFIG_FIXED_PHY for fsl-imx51

BugLink: http://bugs.launchpad.net/bugs/457878

When phylib is enabled in ethernet driver fec.c, there is a sysfs conflict with
this built-in fixed phy driver. Since nobody is using this fixed phy driver in
ARM based machine, it should be turned off.

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Amit Kucheria <amit.kucheria@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>