iommu/arm-smmu: Fix typo in devicetree binding introductory text

This may well be the world's most inconsequential patch, but there is a
spelling mistake that needs fixing and Andrea was bored enough to write
the patch (along with 1528 others...).
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/devicetree/bindings/iommu/arm,smmu-v3.txt

 * ARM SMMUv3 Architecture Implementation
 
-The SMMUv3 architecture is a significant deparature from previous
+The SMMUv3 architecture is a significant departure from previous
 revisions, replacing the MMIO register interface with in-memory command
 and event queues and adding support for the ATS and PRI components of
 the PCIe specification.