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 <[e-mail]>
Cc: Thierry Reding <[e-mail]>
Cc: Joerg Roedel <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: [e-mail]
Reviewed-by: Thierry Reding <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[joro: Improve code formatting]
Signed-off-by: Joerg Roedel <[e-mail]>

drivers/iommu/of_iommu.c

···
 	return ops;
 }
 
-static enum iommu_resv_type iommu_resv_region_get_type(struct device *dev, struct resource *phys,
-						       phys_addr_t start, size_t length)
+static enum iommu_resv_type __maybe_unused
+iommu_resv_region_get_type(struct device *dev,
+			   struct resource *phys,
+			   phys_addr_t start, size_t length)
 {
 	phys_addr_t end = start + length - 1;