dt-bindings: serial: renesas,hscif: Document r8a779g0 bindings

R-Car V4H (R8A779G0) SoC has the R-Car Gen4 compatible HSCIF ports,
so document the SoC specific bindings.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220425064201.459633-4-yoshihiro.shimoda.uh@renesas.com


Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>