media: platform: mdp3: mark OF related data as maybe unused

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

  drivers/media/platform/mediatek/mdp3/mtk-mdp3-comp.c:627:34: error: ‘mdp_sub_comp_dt_ids’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/platform/mediatek/mdp3/mtk-mdp3-comp.c

@@ -746,7 +746,7 @@ static const struct mdp_comp_ops *mdp_comp_ops[MDP_COMP_TYPE_COUNT] = {
 	[MDP_COMP_TYPE_CCORR] =		&ccorr_ops,
 };
 
-static const struct of_device_id mdp_comp_dt_ids[] = {
+static const struct of_device_id mdp_comp_dt_ids[] __maybe_unused = {
 	{
 		.compatible = "mediatek,mt8183-mdp3-rdma",
 		.data = (void *)MDP_COMP_TYPE_RDMA,