iommu/arm-smmu-v3: Remove useless of_match_ptr()

The CONFIG option controlling this driver, ARM_SMMU_V3,
depends on ARM64, which select's OF.

So, CONFIG_OF is always defined when building this driver.
of_match_ptr(arm_smmu_of_match) is the same as arm_smmu_of_match.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Will Deacon <will@kernel.org>