drivers/dma: Rename CAVS driver to DesignWare

There is no such thing as CAVS DMA IP block, the DMA IP block found on
CAVS based chips is made with DesignWare one.

This will help to centralize DW based DMA device into one driver.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>