dt-bindings: arm: Document R-Car M3-N SoC DT bindings

Add device tree bindings documentation for Renesas R-Car M3-N (r8a77965)
SoC.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>