video: exynos_fb: configure backlight GPIOs if specified in DT

Add support to configure PWM_OUT(PWM output) GPIO and
BL_EN(backlight enable) GPIO, if provided in FIMD DT node.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>