mx31moboard: support for the smartbot baseboard

This baseboard is used on the handbot and eybot robots.

The sel gpios are used as enables and rst signals on smartbot, thus the
sel init is moved from mx31moboard file to board files.

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