dt-bindings: clock: r9a06g032-sysctrl: Reference the DMAMUX subnode

dt-bindings: clock: r9a06g032-sysctrl: Reference the DMAMUX subnode

This system controller contains several registers that have nothing to
do with the clock handling, like the DMA mux register. Describe this
part of the system controller as a subnode.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20220427095653.91804-3-miquel.raynal@bootlin.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>