usb: chipidea: imx: allow to configure oc polarity on i.MX25

Up to now the polarity of the over current pin was hard coded to active
high. Use the already defined device tree properties to configure polarity
on i.MX25, too. In difference to i.MX6/7 use active high behavior if the
polarity is unspecified to keep compatibility to existing device trees.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Peter Chen <peter.chen@nxp.com>