ARM: dtsi: imx6qdl-sabresd: Add USB OTG vbus pin to pinctrl_hog

USB OTG vbus pin needs to be configured as gpio function at
sabresd board.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>