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: fc40200e

 ("soc: imx: increase build coverage for imx8m soc driver")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>