backlight: as3711_bl: Include <linux/of.h>

Include <linux/of.h> to declare various OF helpers. Avoids dependency
on backlight header to include it.

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reviewed-by: "Daniel Thompson (RISCstar)" <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/video/backlight/as3711_bl.c

···
 #include <linux/kernel.h>
 #include <linux/mfd/as3711.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/regmap.h>
 #include <linux/slab.h>