crypto: ppc/sha256 - glue

Glue code for crypto infrastructure. Call the assembler
code where required. Disable preemption during calculation
and enable SPE instructions in the kernel prior to the
call. Avoid to disable preemption for too long.

Take a little care about small input data. Kick out early
for input chunks < 64 bytes and replace memset for context
cleanup with simple loop.

Signed-off-by: Markus Stockhausen <stockhausen@collogia.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>