crypto: powerpc/md5 - glue

Glue code for crypto infrastructure. Call the assembler
code where required. 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>