dmaengine: remove DMA_SG as it is dead code in kernel

There are no in kernel consumers for DMA_SG op. Removing operation,
dead code, and test code in dmatest.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Gary Hook <gary.hook@amd.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Cc: Kedareswara rao Appana <appana.durga.rao@xilinx.com>
Cc: Li Yang <leoyang.li@nxp.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/dmaengine/provider.txt

@@ -181,13 +181,6 @@ Currently, the types available are:
     - Used by the client drivers to register a callback that will be
       called on a regular basis through the DMA controller interrupt
 
-  * DMA_SG
-    - The device supports memory to memory scatter-gather
-      transfers.
-    - Even though a plain memcpy can look like a particular case of a
-      scatter-gather transfer, with a single chunk to transfer, it's a
-      distinct transaction type in the mem2mem transfers case
-
   * DMA_PRIVATE
     - The devices only supports slave transfers, and as such isn't
       available for async transfers.