Update the IPMMU DT binding documentation to include the
r8a77970 (R-Car V3M) and r8a77995 (R-Car D3) compat strings.

Based on work for r8a7796 by Magnus Damm.
Signed-off-by: NSimon Horman <horms+renesas@verge.net.au>
Reviewed-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>