exynos: Enable the LCD backlight for snow

The backlight uses FETs on the TPS65090. Enable this so that the display
is visible.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>