ARM: dts: imx6dl-yapp4: Specify USB overcurrent protection polarity

After reset the oc protection polarity is set to active high on imx6.
If the polarity is not specified in device tree it is not changed.

The imx6dl-yapp4 platform uses an active-low oc signal so explicitly
configure that in the device tree.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>