dts: exynos54xx: Add samsung, pwm-out-gpio property to FIMD node

Now that the exynos_fb driver supports handling backlight GPIO
via DT, specify pwm output property via 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>