dma: mv_xor: Add a device_control function

The dmatest module for DMA engines calls

device_control(dtc->chan, DMA_TERMINATE_ALL, 0);

after completing the tests. The documentation in
include/linux/dmaengine.h suggests this function is optional and
dma_async_device_register() also does not BUG_ON() when not passed a
function. However, dmatest is not the only code in the kernel
unconditionally calling device_control. So add an implementation
indicating all operations are not implemented.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>