backlight: lp855x: Mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  drivers/video/backlight/lp855x_bl.c:551:34: error: ‘lp855x_dt_ids’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230311173556.263086-1-krzysztof.kozlowski@linaro.org

drivers/video/backlight/lp855x_bl.c

@@ -548,7 +548,7 @@ static void lp855x_remove(struct i2c_client *cl)
 	sysfs_remove_group(&lp->dev->kobj, &lp855x_attr_group);
 }
 
-static const struct of_device_id lp855x_dt_ids[] = {
+static const struct of_device_id lp855x_dt_ids[] __maybe_unused = {
 	{ .compatible = "ti,lp8550", },
 	{ .compatible = "ti,lp8551", },
 	{ .compatible = "ti,lp8552", },