crypto: omap-sham - Adjust DMA parameters

DMA is set to use burst mode also for source channel. It should
descrease memory bandwidth needs.

DMA synchronization is set to use prefetch mechanism. SHAM block is behind
L4 bus and it doesn't have fifo. SHAM block is stalling as long as the new
data is available. It takes time to fetch data from memory and transfer it
via L4 bus. With prefetch enabled, data is waiting in DMA fifo and SHAM block
receives new data block faster.
This increases SHA processing speed up to 30 percent depending on the
bus / memory load.

Signed-off-by: Samu Onkalo <samu.p.onkalo@nokia.com>
Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@nokia.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>