dmaengine: dmatest: Select DMA_ENGINE_RAID as its needed for the slave_sg test

To enable usage of multiple SG buffers via the sg_buffers= module
parameter, lets select DMA_ENGINE_RAID via Kconfig when DMATEST is
configured. Otherwise the dmatest will "BUG" when more than 1
buffer (total of 2 for src + dst) is configured via sg_buffers.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Kedareswara rao Appana <appanad@xilinx.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>