dt-bindings: arm: renesas: Document R-Car H3e-2G and M3e-2G SoCs and boards

Document the compatible values for the R-Car H3e-2G (R8A779M1) and
M3e-2G (R8A779M3) SoCs.  These are different gradings of the R-Car H3
ES3.0 (R8A77951) and M3-W+ (R8A77961) SoCs.

All R-Car Gen3e on-SoC devices are identical to the devices on the
corresponding R-Car Gen3 SoCs, and thus just use the compatible values
for the latter.  The root compatible properties do gain an additional
value, to sort out integration issues if they ever arise.

Document the use of these SoCs on the Salvator-XS and ULCB (with and
without Kingfisher) development boards.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/fd11328199d8abba5b9ee1f43f4d467d1bc41df6.1626708063.git.geert+renesas@glider.be