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>