ARM: imx/iomux-v3: allow pad_list to be const

Also fix all machine files to make use of it and while at it also make
the pad lists __initconst.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>