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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>