i.MX: Scatter-gather DMA emulation for i.MX/MX1

Patch from Pavel Pisa

This patch contains simplified set of changes to add scatter-gather
emulation capability into MX1 DMA support. The result should
be still usable for next combination of DMA transfers
  Statter-Gather/linear/2D/FIFO to linear/2D/FIFO and
  linear/2D/FIFO to Statter-Gather/2D/FIFO
The patch corrects channel priority allocation to be compatible
with MX1 hardware implementation.
Previous code has not been adapted from its PXA original.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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