iommu/arm-smmu: Fix -Wunused-const-variable warning

For ARCH=arm builds, OF is not necessarily enabled, that is, you can
build this driver without CONFIG_OF.

When CONFIG_OF is unset, of_match_ptr() is NULL, and arm_smmu_of_match
is left orphan.

Building it with W=1 emits a warning:

drivers/iommu/arm-smmu.c:1904:34: warning: ‘arm_smmu_of_match’ defined but not used [-Wunused-const-variable=]
 static const struct of_device_id arm_smmu_of_match[] = {
                                  ^~~~~~~~~~~~~~~~~

There are two ways to fix this:

 - annotate arm_smmu_of_match with __maybe_unused (or surround the
   code with #ifdef CONFIG_OF ... #endif)

 - stop using of_match_ptr()

This commit took the latter solution.

It slightly increases the object size, but it is probably not a big deal
because arm_smmu_device_dt_probe() is also compiled irrespective of
CONFIG_OF.

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