dt-bindings: dma: snps,dw-axi-dmac: Document optional reset

dt-bindings: dma: snps,dw-axi-dmac: Document optional reset

"make dtbs_check":

    Documentation/devicetree/bindings/dma/snps,dw-axi-dmac.yaml
    arch/riscv/boot/dts/canaan/sipeed_maix_bit.dt.yaml: dma-controller@50000000: 'resets' does not match any of the regexes: 'pinctrl-[0-9]+'
	    From schema: Documentation/devicetree/bindings/dma/snps,dw-axi-dmac.yaml

The Synopsys DesignWare AXI DMA Controller on the Canaan K210 SoC
exposes its reset signal.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20211125152008.162571-1-geert@linux-m68k.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>