ARM: imx: Remove iomux-v3 board code

IMX_HAVE_IOMUX_V3 was only used by i.MX25/35 board files. Since the
board files users are gone, it is safe to remove iomux-v3 related
code.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>