rapidio: add DMA engine support for RIO data transfers

Adds DMA Engine framework support into RapidIO subsystem.

Uses DMA Engine DMA_SLAVE interface to generate data transfers to/from
remote RapidIO target devices.

Introduces RapidIO-specific wrapper for prep_slave_sg() interface with an
extra parameter to pass target specific information.

Uses scatterlist to describe local data buffer.  Address flat data buffer
on a remote side.

Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Acked-by: Vinod Koul <vinod.koul@linux.intel.com>
Cc: Li Yang <leoli@freescale.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>