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: NGeert Uytterhoeven <geert@linux-m68k.org>
Acked-by: NRob Herring <robh@kernel.org>
Reviewed-by: NDamien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20211125152008.162571-1-geert@linux-m68k.orgSigned-off-by: NVinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/snps,dw-axi-dmac.yaml

@@ -53,6 +53,9 @@ properties:
     minimum: 1
     maximum: 8
 
+  resets:
+    maxItems: 1
+
   snps,dma-masters:
     description: |
       Number of AXI masters supported by the hardware.