dt-bindings: usb: mtu3: fix typo of DMA clock name

Fix typo of dma_ck
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/5e06482a0be15476c7b5825f155accf98275afa8.1560246390.git.chunfeng.yun@mediatek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/mediatek,mtu3.txt

@@ -16,7 +16,7 @@ Required properties:
 	entry in clock-names
  - clock-names : must contain "sys_ck" for clock of controller,
 	the following clocks are optional:
-	"ref_ck", "mcu_ck" and "dam_ck";
+	"ref_ck", "mcu_ck" and "dma_ck";
  - phys : see usb-hcd.txt in the current directory
  - dr_mode : should be one of "host", "peripheral" or "otg",
 	refer to usb/generic.txt