exynos: usb: Switch USB VBUS GPIOs to be device tree configured

Some Exynos boards, such as the SMDK5250, control USB port power through
a GPIO pin. For now this had been hardcoded in the exynos5-dt board
file, but not all boards use the same pin, requiring local changes to
support different boards.

This patch moves the GPIO initialization into the USB host controller
drivers which they belong to, and uses the samsung,vbus-gpio parameter
in the device tree to configure it.

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>