Fix kernel driver not setting coherent DMA mask

When the kernel driver creates the Ethos-U device, it doesn't setup the
coherent DMA mask. This causes the kernel to generate a warning the
first time any of the coherent DMA functions are used and the kernel
will end up using a default DMA mask.

To ensure that the device uses the correct DMA mask and to no longer get
the warning, the kernel driver will now setup the DMA mask for the
device.

Change-Id: I92c67c85be1754970412da92161dbf1ec993bca3