dt-bindings: arm: renesas: Add M3ULCB with R-Car M3-W+

Add device tree binding documentation for the Renesas M3ULCB board equipped
with an R-Car M3-W+ (R8A77961) SoC.

Signed-off-by: Yuya Hamamachi <yuya.hamamachi.sx@renesas.com>
Link: https://lore.kernel.org/r/20200309064425.25437-2-yuya.hamamachi.sx@renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>