crypto: sha512_generic - add a sha384 0-length pre-computed hash

This patch adds the sha384 pre-computed 0-length hash so that device
drivers can use it when an hardware engine does not support computing a
hash from a 0 length input.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>