mx31moboard: Move usb OTG device registration

In preparation for a new robot which needs the
OTG port as host.

This moves the OTG device registration into board
initialisation.

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