ARM: dts: enable pwm backlight for exynos5250-snow

Add PWM backlight node for exynos5250 based snow board.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>