iommu/ipmmu-vmsa: Include SoC part number in DT binding docs

Add part numbers for APE6 and current set of R-Car Gen2 SoCs to the
IPMMU DT binding documentation. The example is also updated to show
how the generic compatible string may be used as fallback.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>