arm64: dts: imx8mp: Use the correct name for child node "snps, dwc3"

After the node name of "snps,dwc3" has been corrected to start with "usb"
in fsl,imx8mp-dwc3.yaml. Its name in dts should be modified accordingly.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20210329072714.2135-3-thunder.leizhen@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>