dm: backlight: Add a driver for a PWM backlight

Many backlights need to use a PWM to control the brightness. Add a driver
for this. It understands the standard device tree binding.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/video/Makefile b/drivers/video/Makefile
index fa90721..6d89532 100644
--- a/drivers/video/Makefile
+++ b/drivers/video/Makefile
@@ -10,6 +10,9 @@
 obj-$(CONFIG_DM_VIDEO) += backlight-uclass.o
 obj-$(CONFIG_DM_VIDEO) += video-uclass.o vidconsole-uclass.o console_normal.o
 obj-$(CONFIG_DM_VIDEO) += video_bmp.o
+ifdef CONFIG_DM_VIDEO
+obj-$(CONFIG_DM_PWM) += pwm_backlight.o
+endif
 obj-$(CONFIG_VIDEO_ROTATION) += console_rotate.o
 endif