Move platform-mx2-emma to arch/arm/mach-imx/devices and fix the following build
error:

make[2]: *** No rule to make target `arch/arm/mach-imx/devices/platform-mx2-emma.o', needed by `arch/arm/mach-imx/devices/built-in.o'.  Stop.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>