mx31moboard: add support for usb OTG device (v3)

This is used on all board of our system, so again this is done in the
mx31moboard.c file.

changes since v2: call to gpio_request added in platform code, and
moved device initialization in baseboard files.

changes since v1: pins are claimed in another patch

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