mx31moboard: Fix usb PHY reset

Setup the pad with correct pull-up/down before doing the reset.
Assert the PHY enable signal so the reset is really done.

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>