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: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Bryan Wu <cooloney@gmail.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>