ARM: pxa: change SSP DMA channels allocation

ARM: pxa: change SSP DMA channels allocation

Now the dma_slave_map is available for PXA architecture, switch the SSP
device to it.

This specifically means that :
- for platform data based machines, the DMA requestor channels are
  extracted from the slave map, where pxa-ssp-dai.<N> is a 1-1 match to
  ssp.<N>, and the channels are either "rx" or "tx".

- for device tree platforms, the dma node should be hooked into the
  pxa2xx-ac97 or pxa-ssp-dai node.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Daniel Mack <daniel@zonque.org>