ARM: imx: remove mxc_iomux_v3_init() call from imx53_init_early()

Today, imx53 handles iomux configuration using pinctrl driver, so
mxc_iomux_v3_init() call in imx53_init_early() is there for nothing.
Remove the call from there.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>