dmaengine: i.MX: Add support for interleaved transfers.

i.MX2 and i.MX1 chips have the possibility to do
interleaved transfers with two constraints:
- Only one chunk can be used (i.e. only 2D transfers are allowed).
- Only 2 interleaved configurations can be applied at the same time
for all channels.

Since this patch adds a new resource 'slots_2d' which is shared
by all the DMA channels and to avoid disgustin locking BUGs, the
'lock' member has been moved to the global 'imxdma_engine' structure.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>