iommu/mediatek: Drop of_match_ptr to fix -Wunused-const-variable

iommu/mediatek: Drop of_match_ptr to fix -Wunused-const-variable

The of_device_id is included unconditionally by of.h header and used
in the driver as well.  Remove of_match_ptr to fix W=1 compile test
warning with !CONFIG_OF:

    drivers/iommu/mtk_iommu.c:833:34: warning: 'mtk_iommu_of_ids' defined but not used [-Wunused-const-variable=]
      833 | static const struct of_device_id mtk_iommu_of_ids[] = {

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200727181842.8441-1-krzk@kernel.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>