dt-bindings: arm: renesas: Document RZ/G2N SoC DT bindings

Add device tree bindings documentation for the Renesas RZ/G2N (r8a774b1)
SoC.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Link: https://lore.kernel.org/r/1567675844-19247-2-git-send-email-biju.das@bp.renesas.com


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