serial: sh-sci: document R8A77970 bindings

R-Car V3M (R8A77970) SoC also has the R-Car gen3 compatible SCIF and HSCIF
ports, so document the SoC specific bindings.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>