dt-bindings: serial: renesas,sci: Document RZ/G2UL SoC

Add SCI binding documentation for Renesas RZ/G2UL SoC. No driver changes
are required as generic compatible string "renesas,sci" will be used as
a fallback.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20220207142213.19552-3-biju.das.jz@bp.renesas.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>