dt-bindings: power: rcar-sysc: Document r8a774b1 sysc

Document bindings for the RZ/G2N (a.k.a. R8A774B1) system controller.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Link: https://lore.kernel.org/r/1568881036-4404-2-git-send-email-biju.das@bp.renesas.com


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