video/backlight: LP855X needs PWM

The LP855X driver cannot be successfully built if we don't
enable the PWM subsystem. This patch makes that dependency
explicit in Kconfig and prevents broken randconfig builds.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NPeter Griffin <peter.griffin@linaro.org>
Acked-by: NJingoo Han <jg1.han@samsung.com>
Acked-by: NBryan Wu <cooloney@gmail.com>
Acked-by: NMilo Kim <milo.kim@ti.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

drivers/video/backlight/Kconfig

@@ -386,7 +386,7 @@ config BACKLIGHT_LM3639
 
 config BACKLIGHT_LP855X
 	tristate "Backlight driver for TI LP855X"
-	depends on BACKLIGHT_CLASS_DEVICE && I2C
+	depends on BACKLIGHT_CLASS_DEVICE && I2C && PWM
 	help
 	  This supports TI LP8550, LP8551, LP8552, LP8553, LP8555, LP8556 and
 	  LP8557 backlight driver.