crypto: padlock-sha - Add import/export support

As the padlock driver for SHA uses a software fallback to perform
partial hashing, it must implement custom import/export functions.
Otherwise hmac which depends on import/export for prehashing will
not work with padlock-sha.

Reported-by: Wolfgang Walter <wolfgang.walter@stwm.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>