ARM: mxs: Enable MACH_APX4DEVKIT

Enable MACH_APX4DEVKIT to be built by default.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Lauri Hintsala <lauri.hintsala@bluegiga.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>