dt-bindings: iommu: renesas,ipmmu-vmsa: convert to json-schema

Convert Renesas VMSA-Compatible IOMMU bindings documentation
to json-schema.

Note that original documentation doesn't mention renesas,ipmmu-vmsa
for R-Mobile APE6. But, R-Mobile APE6 is similar to the R-Car
Gen2. So, renesas,ipmmu-r8a73a4 belongs the renesas,ipmmu-vmsa
section.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>