snippets: xen_dom0: add support for Renesas R-Car Gen3 boards

Renesas rcar_h3ulcb_ca57 and rcar_salvator_xs_m3 boards support recently
was added to Zephyr mainline. Add Xen control domain guest support for
them via xen_dom0 snippet extention.

Note: please pay attetion to overlay nodes and comments in case of any
issues on your setup.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>