dt-bindings: arm: renesas: Add R-Car M3-N ULCB with Kingfisher

Document the use of the Kingfisher expansion board with the R-Car
Starter Kit Pro equipped with an R-Car M3-N SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20191016151109.30747-1-geert+renesas@glider.be