ARM: imx: move iomux drivers and headers into mach-imx

The board files in mach-imx are the only users of iomux drivers and
headers.  Move them into mach-imx from plat-mxc.

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