crypto: sha512-s390 - Add export/import support

This patch adds export/import support to sha512-s390 (which includes
sha384-s390).  The exported type is defined by struct sha512_state,
which is basically the entire descriptor state of sha512_generic.

Since sha512-s390 only supports a 64-bit byte count the import
function will reject anything that exceeds that.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>