mx31moboard: use of new iomux implementation

This example takes advantage of the possibility to use tables of iomux
configs.
This is inspired from mx1-mx2 iomux code. It allows a better code
readability.

Signed-off-by: Valentin Longchamp <valentin.longchamp@epfl.ch>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>