ehci-mxc: 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.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>