ARM: imx: make ehci power/oc polarities configurable

Make ehci power and overcurrent polarities configurable. If not set, these new
configurartions keep the default register values so that existing board files
do not have to be changed.

Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: <linux-arm-kernel@lists.infradead.org>
Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>