dt-bindings: arm: toshiba: Add the TMPV7708 RM main board

Add an entry for the Toshiba Visconti TMPV7708 RM main board
(tmpv7708-rm-mbrc) to the board/SoC bindings.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Reviewed-by: Punit Agrawal <punit1.agrawal@toshiba.co.jp>
Reviewed-by: Rob Herring <robh@kernel.org>