dt-bindings: clock: renesas,r9a06g032-sysctrl: documentation

The Renesas R9A06G032 SYSCTRL node description.

Signed-off-by: Michel Pollet <michel.pollet@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>