iommu/ipmmu-vmsa: Add r8a7795 DT binding

Update the IPMMU DT binding documentation to include the r8a7795 compat
string as well as the "renesas,ipmmu-main" property that on r8a7795 will
be used to describe the topology and the relationship between the various
cache IPMMU instances and the main IPMMU.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Joerg Roedel <jroedel@suse.de>