crypto: img-hash - Reconfigure DMA Burst length

Burst length of 16 drives the hash accelerator out of spec
and causes stability issues in some cases. Reduce this to
stop data being lost.

Signed-off-by: Will Thomas <will.thomas@imgtec.com>
Reviewed-by: James Hartley <james.hartley@imgtec.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>