soc: imx8m: fix build warning

Fix the build warning with x86_64-randconfig
>> drivers/soc/imx/soc-imx8m.c:150:34: warning: unused variable
>> 'imx8_soc_match' [-Wunused-const-variable]
static const struct of_device_id imx8_soc_match[] = { ^

Fixes: fc40200ebf82 ("soc: imx: increase build coverage for imx8m soc driver")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Peng Fan <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

drivers/soc/imx/soc-imx8m.c

···
 	.soc_revision = imx8mm_soc_revision,
 };
 
-static const struct of_device_id imx8_soc_match[] = {
+static __maybe_unused const struct of_device_id imx8_soc_match[] = {
 	{ .compatible = "fsl,imx8mq", .data = &imx8mq_soc_data, },
 	{ .compatible = "fsl,imx8mm", .data = &imx8mm_soc_data, },
 	{ .compatible = "fsl,imx8mn", .data = &imx8mn_soc_data, },