usb: chipidea: imx: Warn if oc polarity isn't specified

The polarity of the over current detection pin isn't configured on i.MX6/7
if it's unspecified in the device tree. So the actual configuration depends
on bootloader behavior which is bad.

So encourage users to fix their device tree by issuing a warning in this
case.

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