crypto: sha256 - Add sha224 support to sha256 library code

Add sha224 support to the lib/crypto/sha256 library code. This will allow
us to replace both the sha256 and sha224 parts of crypto/sha256_generic.c
when we remove the code duplication in further patches in this series.

Suggested-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>