dm: backlight: Add CONFIG_BACKLIGHT_PWM

Add a config to select individually the driver for PWM backlights.
Manage "depends on" to be backyard compatible.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/drivers/video/Makefile b/drivers/video/Makefile
index 5cf8909..fc1cd14 100644
--- a/drivers/video/Makefile
+++ b/drivers/video/Makefile
@@ -11,9 +11,7 @@
 obj-$(CONFIG_DM_VIDEO) += panel-uclass.o simple_panel.o
 obj-$(CONFIG_DM_VIDEO) += video-uclass.o vidconsole-uclass.o
 obj-$(CONFIG_DM_VIDEO) += video_bmp.o
-ifdef CONFIG_DM_VIDEO
-obj-$(CONFIG_DM_PWM) += pwm_backlight.o
-endif
+obj-$(CONFIG_BACKLIGHT_PWM) += pwm_backlight.o
 obj-$(CONFIG_CONSOLE_NORMAL) += console_normal.o
 obj-$(CONFIG_CONSOLE_ROTATION) += console_rotate.o
 obj-$(CONFIG_CONSOLE_TRUETYPE) += console_truetype.o fonts/