ARM: imx: include board headers in the same folder

The headers that are only used by board files do not necessarily
need to be in plat-mxc/include/mach.  Move them to the same place
as those board files.

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