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>