video/backlight: LP8788 needs PWM

The LP8788 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.

Based on Arnd Bergmann patch but split out into seperate
commits per driver based on feedback.

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>