iommu/of: mark an unused function as __maybe_unused

iommu/of: mark an unused function as __maybe_unused

When CONFIG_OF_ADDRESS is not set, there is a build warning/error
about an unused function.
Annotate the function to quieten the warning/error.

../drivers/iommu/of_iommu.c:176:29: warning: 'iommu_resv_region_get_type' defined but not used [-Wunused-function]
  176 | static enum iommu_resv_type iommu_resv_region_get_type(struct device *dev, struct resource *phys,
      |                             ^~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: a5bf3cfce8cb ("iommu: Implement of_iommu_get_resv_regions()")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: iommu@lists.linux.dev
Reviewed-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20230209010359.23831-1-rdunlap@infradead.org
[joro: Improve code formatting]
Signed-off-by: Joerg Roedel <jroedel@suse.de>