perf/marvell_cn10k: Fix unused variable warning when W=1 and CONFIG_OF=n

perf/marvell_cn10k: Fix unused variable warning when W=1 and CONFIG_OF=n

The kbuild helpfully reports that the Marvell CN10K TAD PMU driver emits
a warning when building with W=1 and CONFIG_OF=n:

  | >> drivers/perf/marvell_cn10k_tad_pmu.c:371:34: warning: unused variable 'tad_pmu_of_match' [-Wunused-const-variable]
       static const struct of_device_id tad_pmu_of_match[] = {

Guard the match table with CONFIG_OF to squash the warning.

Link: https://lore.kernel.org/r/202201292349.zRQLcDDD-lkp@intel.com
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Will Deacon <will@kernel.org>