dts: bcm2712: put usb under /axi not /soc

On 2712, the DWC USB controller is no longer attached to the Videocore
30-bit bus with its associated aliases, and can see the bottom 4GB of
RAM directly.

Ideally it should make use of IOMMU6 but for now software bounce buffers
get it working.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>