ARM: imx: move platform device code into mach-imx

It moves platform device code from plat-mxc into mach-imx.  Along with
that, header devices-common.h gets moved from plat-mxc/include/mach/
into mach-imx/devices/.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>