iommu/ipmmu-vmsa: Add r8a7796 DT binding

Update the IPMMU DT binding documentation to include the r8a7796 compat
string for R-Car M3-W.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>