ARM: imx: iomux: Do not export symbol without public declaration

The iomux function declarations are in headers only accessible in this
directory. Thus those can't be used in any module. None of the
objects in this directory is tristate. Neither can the header be included
in out-of-tree modules.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>