dt-bindings: power: rcar-sysc: Document r8a774c0 sysc

Document bindings for the RZ/G2E (a.k.a. R8A774C0) system
controller.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>