Docs: dt: Be explicit and consistent in reference to IOMMU specifiers

The generic IOMMU binding says that the meaning of an 'IOMMU specifier'
is defined by the binding of a specific SMMU.  The ARM SMMU binding
never explicitly uses the term 'specifier' at all.  Update implicit
references to use the explicit term.

In the iommu-map binding change references to iommu-specifier to
"IOMMU specifier" so we are 100% consistent everywhere with terminology
and capitalization.

Signed-off-by: Stuart Yoder <stuart.yoder@nxp.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>