dt-bindings: serial: renesas,em-uart: Document r9a09g011 bindings

The Renesas RZ/V2M (r9a09g011) SoC uses a uart that is compatible with the
EMMA Mobile SoC.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20220330154024.112270-3-phil.edworthy@renesas.com
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>