usb: chipidea: imx: support configuring for active low oc signal

The status quo on i.MX6 is that if "over-current-active-high" is
specified in the device tree this is configured as expected. If
the property is missing polarity isn't changed and so the
polarity is kept as setup by the bootloader. Reset default is
active high, so active low can only be used with help by the
bootloader. On i.MX7 it is similar, but there disabling of
over current detection has a similar inconsistency.

This patch introduces a new property that allows to explicitly
configure for active low over current detection and consistently
sets this up. In the absence of an explicit configuration the
bit is kept as is. On i.MX7 over current detection is used unless
disabled in the device tree.

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