ARM: efikamx: remove Genesi Efika MX from the i.MX v6/v7 defconfig

No need to have Efika MX listed in the defconfig if it can't be built.

Signed-off-by: Matt Sealey <matt@genesi-usa.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>