exynos: dts: Add USB VBUS GPIOs to the device tree

This patch adds a new samsung,vbus-gpio parameter to the device tree, in
preparation of replacing the currently hardcoded VBUS GPIO mechanism in
exynos5-dt.c with a device tree controlled solution, just as it already
exists in the Linux kernel.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Marek Vasut <marex@denx.de>