mx31: define various pins used on mx31moboard (v2)

As suggested by Sascha, I regroup them in a single patch so that the
other patches become more orthogonal.

changes since v1: changed I2C1 pin names

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