usb: chipidea: imx: set power polarity

This patch adds support to set the power line polarity for i.MX SoCs.

To let the USB controller control the power it may be necessary to
configure the polarity of the power line. So far the polarity was
configured by Bootloader or alternatively the power line was muxed
as gpio and driven by a regulator.

Also make use of of_property_read_bool.

Signed-off-by: Philipp Puschmann <philipp.puschmann@emlix.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>