caam_read copies random bytes from two buffers into output.

caam rng can fill empty buffer 0xffff bytes at a time,
but the buffer sizes are rounded down to multiple of cacheline size.
Signed-off-by: Yuan Kang <Yuan.Kang@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>