dt-bindings: serial: sh-sci: Document r8a7778/9 HSCIF bindings

Document support for the HSCIF serial ports in the Renesas R-Car M1A
(R8A7778) and H1 (R8A7779) SoCs.

No driver update is needed.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
[geert: Patch description]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>